feat(ui): phone layout with a bottom tab bar and one panel sheet at a time

At 720px and narrower the globe stays full screen and a bottom tab bar (Layers,
Cameras, Context, Search, More) opens one bottom sheet at a time, presenting the
existing rail and dock panels in place. Sheets borrow panels without persisting
or sharing them and put back what they changed. Cockpit, recording and clean
view close the sheet; scene playback keeps STOP reachable. Focus moves into the
sheet and back to its tab, the required attribution stays visible above the
voice control and any sheet, and the page uses safe-area insets. Wider
viewports are unchanged.

Adapted from the phone tab-shell proposed in #203 by @blearymoth.

src/ui/rightPanelRail.js

@@ -91,7 +91,9 @@ export function layoutRightPanelRail({
   }
   // Normalize restored state and theme entry without overwriting saved panel
   // preferences. This marker is distinct from Tactical's space-based collapse.
-  if (hud.variant === 'cyber') {
+  // Phones present the rail as tab sheets that show several panels at once.
+  const isMobile = windowRef.matchMedia('(max-width: 720px)').matches;
+  if (hud.variant === 'cyber' && !isMobile) {
     const expanded = panels.filter(
       (panel) => !panel.classList.contains('collapsed'),
     );
@@ -111,16 +113,19 @@ export function layoutRightPanelRail({
       onCollapse(panel);
     }
   }
-  const isMobile = windowRef.matchMedia('(max-width: 720px)').matches;
   const hasExpandedPanel = panels.some(
     (panel) =>
       !panel.classList.contains('collapsed') &&
       (!isMobile || panel.id !== 'pp-toggles'),
   );
-  const exclusive = shouldHideCollapsedRightPanels({
-    hudVariant: hud.variant,
-    hasExpandedPanel,
-  });
+  // Phones present the rail as tab sheets (mobileShell.js), which choose what
+  // shows; hiding collapsed siblings would empty a sheet and its a11y tree.
+  const exclusive =
+    !isMobile &&
+    shouldHideCollapsedRightPanels({
+      hudVariant: hud.variant,
+      hasExpandedPanel,
+    });
   stack.classList.toggle('layout-exclusive', exclusive);
   for (const panel of panels) {
     if (exclusive && panel.classList.contains('collapsed'))