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/frontend/src/components/mock-layout/ui.tsx

@@ -205,4 +205,6 @@ export const PanelHeaderBar = styled("div", ({ $theme }) => ({
   backgroundColor: $theme.colors.backgroundTertiary,
   gap: "8px",
   flexShrink: 0,
+  position: "relative" as const,
+  zIndex: 9999,
 }));