feat(share): short /share/<code> links with a compact versioned view format (#1418)

* feat(share): add compact versioned share-view codec

Add a standalone codec for `#s=<base64url>` share links: a version byte
followed by protobuf-style tag-length-value fields. Settings equal to
their default are omitted and refilled from the same table on decode;
unknown fields are skipped by wire type, so new fields never break
older apps. Views are a place (camera) or a thing (an aircraft,
military aircraft, vessel or satellite to follow, with camera offset
and last-known fix); an unknown kind falls back to the camera.

The decoder bounds its input and rejects malformed payloads with a
reason instead of throwing. A converter maps the long form's parsed
state to and from the codec state; the long form is unchanged.

* fix(share): tighten codec decoding after review

Unknown detection mode and allocation names now fall back to their
defaults instead of reaching the lenient long-form migration. Future
panel values and style-parameter tokens are skipped rather than applied
or rejected, and layer options are accepted only under their owner
layer. Wrap longitudes before scaling, normalize negative zero, reuse
the layer-state tracking ID grammar, and align the format doc with the
decoder.

* feat(share): let a host share the link its own way

ShareLinkManager.buildLink() returns the current share link without
copying it, and copyLink() now copies exactly that. The share button
passes the link to an optional services.shareLink(url), which resolves
to 'shared', 'copied', 'cancelled' or false; 'cancelled' shows no toast
and 'shared' shows the new "Link shared" (es, ru, zh-CN). Without the
service the button copies as before.

* feat(share): open and make short /share/<code> links

The share button now makes `<app>/share/<code>` links with the compact
share-view code, and the app opens them at start-up through the same
restore pipeline as long links. Once the app next writes the address, the
share path is replaced (no new history entry) by the app's own address and
the live long-form hash, so a reload follows the live view. A host can pass
the code as `initialShareCode` instead. A code that fails to decode restores
nothing and shows "This share link can't be opened".

The code gains a trailing CRC-8 check byte, so any single mistyped
character is rejected instead of opening a different view. Thing views no
longer store a camera: it is derived from the entity's last-known fix and
the follow offset on a sphere, the entity kind is implied by the id's
field, and the followed entity's layer bit is implied; an aircraft followed
from the app's own camera is a 42-character code. The `#s=` hash form is
gone.

While the camera follows an aircraft, military aircraft or satellite, the
link is a thing view with the camera's offset and the entity's position.
Opening one turns the layer on and follows the entity through the existing
shared-follow restore, then swings the camera round to the shared offset; a
vessel view selects the vessel once the vessel feed has it. If the entity
is not reporting within its window, the view stays at its last-known fix
and says so. The panel and style-parameter tables now come from the codec,
and a host share hook that returns a plain value or throws still gets a
toast.

* fix(share): encode style settings against the style's own look

Settings a style applies when selected (bloom, sharpen, HUD, detection,
celestial ring and the style's shader parameters) now default, per field,
to what that style applies in a fresh session, read from the tables the
app applies (the first-load baseline, the style presets and the shader
defaults). The encoder compares against the view's style and the decoder
fills omitted settings from the decoded style, so a view that keeps its
style's look costs only its camera and style. The previous defaults did not
match a fresh session (sharpen, HUD and detection were off), so every link
paid for them, and a style's own shader parameters were always written.

A test pins the per-style defaults as frozen v1 meanings. A followed
aircraft in Noir is now 44 characters and the real long-link example 39.

* docs: changelog for #1418

src/ui/shareFollow.js

@@ -0,0 +1,190 @@
+import * as Cesium from 'cesium';
+import { trackedDisplayPositionForCamera } from '../data/trackedCamera.js';
+import {
+  SHARE_VIEW_ENTITY_KINDS,
+  followOffsetFromEnu,
+  followOffsetToEnu,
+} from '../share/follow.js';
+
+/**
+ * The bridge between the camera following an entity and a share link's thing
+ * view: what the camera follows now, in the share format's terms, and the
+ * stored follow offset applied to an entity the camera has started to follow.
+ */
+
+const KIND_BY_LAYER = new Map(
+  SHARE_VIEW_ENTITY_KINDS.map((entry) => [entry.layerId, entry.kind]),
+);
+const MAX_FRAMES = 120;
+
+/** `position`'s local east/north/up offset from `origin`, in metres. */
+function enuOffset(origin, position) {
+  const frame = Cesium.Transforms.eastNorthUpToFixedFrame(origin);
+  const local = Cesium.Matrix4.multiplyByPoint(
+    Cesium.Matrix4.inverseTransformation(frame, new Cesium.Matrix4()),
+    position,
+    new Cesium.Cartesian3(),
+  );
+  return { east: local.x, north: local.y, up: local.z };
+}
+
+/**
+ * The entity the camera follows as `{ kind, id, offset, lastKnown }`, with
+ * the camera's offset from it and its displayed position as of `nowMs`; null
+ * when the camera follows nothing a share link can name. A selected vessel is
+ * not followed (the camera stays where the user puts it), so a link made then
+ * keeps its own camera.
+ */
+export function readShareFollowSubject({ viewer, nowMs = Date.now() }) {
+  const cameraPosition = viewer?.camera?.positionWC;
+  if (!cameraPosition) return null;
+  const tracked = viewer.trackedEntity;
+  const trackedId = String(tracked?.gevTrackedId || '');
+  const separator = trackedId.indexOf(':');
+  if (separator <= 0) return null;
+  const kind = KIND_BY_LAYER.get(trackedId.slice(0, separator));
+  const id = trackedId.slice(separator + 1);
+  const target = kind
+    ? trackedDisplayPositionForCamera(
+        tracked,
+        viewer.clock.currentTime,
+        new Cesium.Cartesian3(),
+      )
+    : null;
+  if (!kind || !id || !target) return null;
+  const offset = followOffsetFromEnu(enuOffset(target, cameraPosition));
+  const fix = Cesium.Cartographic.fromCartesian(target);
+  if (!offset || !fix) return null;
+  return {
+    kind,
+    id,
+    offset,
+    lastKnown: {
+      lat: Cesium.Math.toDegrees(fix.latitude),
+      lon: Cesium.Math.toDegrees(fix.longitude),
+      alt: fix.height,
+      observedAtMs: nowMs,
+    },
+  };
+}
+
+/** Largest turn, in radians, the camera makes around the entity per frame. */
+const MAX_TURN_PER_FRAME = Math.PI / 3;
+
+/**
+ * Put the camera at a shared follow offset from `entity` once following it
+ * has framed the camera. Cesium keeps the offset afterwards, as it keeps one
+ * the user orbits to. The camera swings round the entity at most 60° a frame:
+ * the follow's own guard treats a jump to the far side as the camera crossing
+ * through the entity and puts it back. Returns a disposer.
+ */
+export function applyShareFollowOffset(viewer, entity, offset) {
+  if (!viewer?.scene?.preUpdate || !entity || !(offset?.range > 0))
+    return () => {};
+  const local = followOffsetToEnu(offset);
+  let frames = 0;
+  let remove = null;
+  const stop = () => {
+    remove?.();
+    remove = null;
+  };
+  remove = viewer.scene.preUpdate.addEventListener(() => {
+    if (viewer.isDestroyed?.() || viewer.trackedEntity !== entity) {
+      stop();
+      return;
+    }
+    frames += 1;
+    const camera = viewer.camera;
+    // Let the follow frame the camera first (its first frame), then move it.
+    const framed =
+      frames > 1 &&
+      !Cesium.Matrix4.equals(camera.transform, Cesium.Matrix4.IDENTITY);
+    const target = framed
+      ? trackedDisplayPositionForCamera(
+          entity,
+          viewer.clock.currentTime,
+          new Cesium.Cartesian3(),
+        )
+      : null;
+    if (!target) {
+      if (frames >= MAX_FRAMES) stop();
+      return;
+    }
+    // The offset is east/north/up at the entity; the follow's own frame may
+    // differ, so place the camera in world space and re-express it there.
+    const world = Cesium.Matrix4.multiplyByPoint(
+      Cesium.Transforms.eastNorthUpToFixedFrame(target),
+      new Cesium.Cartesian3(local.east, local.north, local.up),
+      new Cesium.Cartesian3(),
+    );
+    const goal = Cesium.Matrix4.multiplyByPoint(
+      Cesium.Matrix4.inverseTransformation(
+        camera.transform,
+        new Cesium.Matrix4(),
+      ),
+      world,
+      new Cesium.Cartesian3(),
+    );
+    const from = Cesium.Cartesian3.clone(
+      camera.position,
+      new Cesium.Cartesian3(),
+    );
+    const angle =
+      Cesium.Cartesian3.magnitude(from) > 0
+        ? Cesium.Cartesian3.angleBetween(from, goal)
+        : 0;
+    const done = !(angle > MAX_TURN_PER_FRAME);
+    const next = done ? goal : turnToward(from, goal, MAX_TURN_PER_FRAME);
+    camera.lookAtTransform(camera.transform, next);
+    viewer.scene.requestRender?.();
+    if (done) stop();
+  });
+  return stop;
+}
+
+/**
+ * `from` turned `step` radians toward `to`, with its length moved the same
+ * fraction of the way: one frame of the swing round the entity.
+ */
+function turnToward(from, to, step) {
+  const angle = Cesium.Cartesian3.angleBetween(from, to);
+  const unitFrom = Cesium.Cartesian3.normalize(from, new Cesium.Cartesian3());
+  // The direction perpendicular to `from` in the plane toward `to` (or any
+  // perpendicular when they are opposite).
+  let toward = Cesium.Cartesian3.subtract(
+    Cesium.Cartesian3.normalize(to, new Cesium.Cartesian3()),
+    Cesium.Cartesian3.multiplyByScalar(
+      unitFrom,
+      Math.cos(angle),
+      new Cesium.Cartesian3(),
+    ),
+    new Cesium.Cartesian3(),
+  );
+  if (Cesium.Cartesian3.magnitude(toward) < 1e-9)
+    toward = Cesium.Cartesian3.cross(
+      unitFrom,
+      Math.abs(unitFrom.z) < 0.9
+        ? Cesium.Cartesian3.UNIT_Z
+        : Cesium.Cartesian3.UNIT_X,
+      new Cesium.Cartesian3(),
+    );
+  Cesium.Cartesian3.normalize(toward, toward);
+  const length =
+    Cesium.Cartesian3.magnitude(from) +
+    (Cesium.Cartesian3.magnitude(to) - Cesium.Cartesian3.magnitude(from)) *
+      (step / angle);
+  const direction = Cesium.Cartesian3.add(
+    Cesium.Cartesian3.multiplyByScalar(
+      unitFrom,
+      Math.cos(step),
+      new Cesium.Cartesian3(),
+    ),
+    Cesium.Cartesian3.multiplyByScalar(
+      toward,
+      Math.sin(step),
+      new Cesium.Cartesian3(),
+    ),
+    new Cesium.Cartesian3(),
+  );
+  return Cesium.Cartesian3.multiplyByScalar(direction, length, direction);
+}