feat(surface): switch app shells when the viewport crosses the phone threshold

The mobile-vs-desktop surface is stamped once at boot, so a browser
window narrowed past the phone threshold kept the desktop shell (and
its legacy squeezed layout) until a manual reload. A viewport watcher
now reloads into the other shell once the resize settles — the same
mechanism the old Settings toggle used. Fixed shells (Capacitor,
desktop, VS Code) and ?surface= overrides never switch.

With the new mobile app reachable this way, the old/new mobile layout
preference is gone: phones always get the mobile app.

packages/ui/src/lib/i18n/messages/de.settings.ts

@@ -1845,9 +1845,6 @@ export const settingsDict = {
   'settings.voice.page.field.ttsInputModeRaw': 'Rohes Markdown',
   'settings.voice.page.field.ttsInputModeSummarized': 'zusammengefasst',
   'settings.openchamber.visual.section.colorMode': 'Farbmodus',
-  'settings.openchamber.visual.section.mobileLayout': 'Mobiles Layout',
-  'settings.openchamber.visual.option.mobileLayout.default': 'Alt',
-  'settings.openchamber.visual.option.mobileLayout.new': 'Neu',
   'settings.openchamber.visual.section.localization': 'Lokalisierung',
   'settings.openchamber.visual.section.spacingAndLayout': 'Abstand & Layout',
   'settings.openchamber.visual.section.navigation': 'Navigation',