Add Foundry mobile layout with Tauri iOS/Android support

- Add responsive mobile layout with bottom pill tab bar, swipe navigation, and task list as home screen
- Add platform detection (useIsMobile hook) with viewport breakpoint and VITE_MOBILE build flag
- Mobile-optimize settings/billing/account pages (single-column layout with horizontal tabs)
- Add iOS safe area inset handling with 47px minimum padding
- Scaffold Tauri v2 mobile targets (iOS/Android) with platform-gated sidecar and capabilities
- Add notification sound support and mobile build script

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

foundry/packages/desktop/src-tauri/capabilities/default.json

@@ -2,6 +2,7 @@
   "identifier": "default",
   "description": "Default capability for Foundry desktop",
   "windows": ["main"],
+  "platforms": ["macOS", "windows", "linux"],
   "permissions": [
     "core:default",
     "core:window:allow-start-dragging",