feat: return the view that shows each answer

Tool answers with something to show now include data.view: the view with
the matching layers on, the area framed from above or a single aircraft or
satellite followed, and the link that opens it. Area framing moves to
src/tools/views.js so links and answers frame alike.

src/tools/views.js

@@ -0,0 +1,55 @@
+/**
+ * Suggested views: the God's Eye View view that shows a tool's answer, and
+ * the link that opens it when the app's address is known.
+ */
+
+import { createView, viewUrl } from '../view/index.js';
+import { areaCenter, areaRadiusKm } from './area.js';
+
+const MIN_ALTITUDE_M = 500;
+const MAX_ALTITUDE_M = 15_000_000;
+// A top-down view sees roughly this many meters of ground per meter of altitude.
+const GROUND_PER_ALTITUDE = 0.55;
+const POINT_ALTITUDE_M = 50_000;
+
+/** A camera looking straight down that frames a resolved area. */
+export function cameraForArea(area) {
+  const center = areaCenter(area);
+  return {
+    lat: center.lat,
+    lon: center.lon,
+    altitude_m: Math.min(
+      MAX_ALTITUDE_M,
+      Math.max(
+        MIN_ALTITUDE_M,
+        (areaRadiusKm(area) * 1000) / GROUND_PER_ALTITUDE,
+      ),
+    ),
+  };
+}
+
+/** The app's address, or null when it is not configured. */
+function appUrl(services) {
+  try {
+    return services.app?.baseUrl ? new URL(services.app.baseUrl).href : null;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * A view of an area or point with layers on and optionally something
+ * followed, plus `url` (null without the app's address). A point is framed
+ * from `altitudeM` above it.
+ */
+export function suggestView(
+  services,
+  { area, point, layers = [], follow = null, altitudeM = POINT_ALTITUDE_M },
+) {
+  const camera = area
+    ? cameraForArea(area)
+    : { lat: point.lat, lon: point.lon, altitude_m: altitudeM };
+  const view = createView({ camera, layers, follow });
+  const base = appUrl(services);
+  return { ...view, url: base ? viewUrl(base, view) : null };
+}