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/src-tauri/gen/schemas/capabilities.json

@@ -0,0 +1 @@
+{"default":{"identifier":"default","description":"Default capability for Foundry desktop","local":true,"windows":["main"],"permissions":["core:default","core:window:allow-start-dragging","shell:allow-open",{"identifier":"shell:allow-execute","allow":[{"args":true,"name":"sidecars/foundry-backend","sidecar":true}]},{"identifier":"shell:allow-spawn","allow":[{"args":true,"name":"sidecars/foundry-backend","sidecar":true}]}]}}