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>

justfile

@@ -184,6 +184,14 @@ foundry-hf *ARGS:
 foundry-docker-build tag='foundry:local':
 	docker build -f foundry/Dockerfile -t {{tag}} .
 
+[group('foundry')]
+foundry-desktop-dev:
+	pnpm --filter @sandbox-agent/foundry-desktop dev
+
+[group('foundry')]
+foundry-desktop-build:
+	pnpm --filter @sandbox-agent/foundry-desktop build:all
+
 [group('foundry')]
 foundry-railway-up:
 	npx -y @railway/cli up --detach