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/firstRunExperience.js

@@ -22,6 +22,7 @@ import { createSurfaceKeyboard } from './ui/surfaceKeyboard.js';
 // is enthusiasm, not "never show me this again".
 
 import { t, msg } from './i18n/index.js';
+import { EXCLUSIVE_SURFACE_CLASSES } from './exclusiveSurfaces.js';
 
 /** Durable suppression. Written ONLY by the "Don't show this again" checkbox. */
 export const FIRST_RUN_STORAGE_KEY = 'gev:first-run-mission:v1';
@@ -305,12 +306,7 @@ export async function runFirstRunChoice(
  * hides the launcher in CSS, and the observer below turns that into a yield —
  * see the ESC ARBITRATION note on initFirstRunExperience.
  */
-export const EXCLUSIVE_SURFACE_CLASSES = Object.freeze([
-  'cockpit-mode',
-  'scene-playback-mode',
-  'recording-mode',
-  'ui-clean-view',
-]);
+export { EXCLUSIVE_SURFACE_CLASSES };
 
 /**
  * Is some other surface currently claiming the screen?