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/tauri.android.conf.json

@@ -0,0 +1,10 @@
+{
+  "$schema": "https://raw.githubusercontent.com/tauri-apps/tauri/dev/crates/tauri-cli/schema.json",
+  "build": {
+    "beforeDevCommand": "FOUNDRY_FRONTEND_CLIENT_MODE=mock VITE_MOBILE=1 pnpm --filter @sandbox-agent/foundry-frontend dev --host 0.0.0.0 --port 4173"
+  },
+  "bundle": {
+    "externalBin": []
+  },
+  "plugins": {}
+}