Add Foundry Tauri v2 desktop app with UI polish

- Scaffold Tauri v2 desktop package (foundry/packages/desktop)
- Sidecar build script compiles backend into standalone Bun binary
- Frontend build script packages Vite output for Tauri webview
- macOS glass-effect app icon following Big Sur design standards
- Collapsible sidebars with smooth width transitions
- Inset content framing with borders and nested border-radius (Outer R = Inner R + Padding)
- iMessage-style chat bubble styling with proper corner radii
- Styled composer input with matching border-radius
- Vertical separator between chat and right sidebar
- Website download button component
- Cargo workspace exclude for standalone Tauri build

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

foundry/packages/desktop/package.json

@@ -0,0 +1,22 @@
+{
+  "name": "@sandbox-agent/foundry-desktop",
+  "version": "0.1.0",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "tauri dev",
+    "build": "tauri build",
+    "build:sidecar": "tsx scripts/build-sidecar.ts",
+    "build:frontend": "tsx scripts/build-frontend.ts",
+    "build:all": "pnpm build:sidecar && pnpm build:frontend && pnpm build",
+    "tauri": "tauri"
+  },
+  "devDependencies": {
+    "@tauri-apps/cli": "^2",
+    "tsx": "^4"
+  },
+  "dependencies": {
+    "@tauri-apps/api": "^2",
+    "@tauri-apps/plugin-shell": "^2"
+  }
+}