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>

.gitignore

@@ -55,3 +55,7 @@ Cargo.lock
 
 # CLI binaries (downloaded during npm publish)
 sdks/cli/platforms/*/bin/
+
+# Foundry desktop app build artifacts
+factory/packages/desktop/frontend-dist/
+factory/packages/desktop/src-tauri/sidecars/