feat: integrate Tauri v2 with Android widget and voice assistant

- Add Tauri v2 shell (Cargo, tauri.conf.json, capabilities, plugins)
- Migrate all fetch() calls to apiFetch() for Tauri-aware HTTP
- Migrate WebSocket endpoints to resolveEndpoints() for dynamic URLs
- Add ServerConfigDialog for remote server URL configuration
- Add tauri.ts lib with isTauri detection, apiFetch wrapper, plugin helpers
- Add server-config Pinia store with persistence via plugin-store
- Conditional PWA (disabled in Tauri builds)
- Android: home screen transcript widget (last 5 messages, 30s refresh)
- Android: voice command / share activity (SpeechRecognizer + WebSocket)
- Android: signed release APK with auto-copy to installers/
- Remove stale frontend/src-tauri directory

src-tauri/tauri.conf.json

@@ -0,0 +1,39 @@
+{
+  "$schema": "https://raw.githubusercontent.com/tauri-apps/tauri/dev/crates/tauri-config-schema/schema.json",
+  "productName": "Agent UI",
+  "version": "0.1.0",
+  "identifier": "com.agentui.desktop",
+  "build": {
+    "frontendDist": "../frontend/dist",
+    "devUrl": "http://localhost:4100",
+    "beforeDevCommand": "cd frontend && npm run dev",
+    "beforeBuildCommand": "cd frontend && npx vite build"
+  },
+  "app": {
+    "windows": [
+      {
+        "title": "Agent UI",
+        "width": 1280,
+        "height": 800,
+        "minWidth": 800,
+        "minHeight": 600,
+        "decorations": true,
+        "resizable": true
+      }
+    ],
+    "security": {
+      "csp": null
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns",
+      "icons/icon.ico"
+    ]
+  }
+}