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>

frontend/packages/website/src/pages/index.astro

@@ -5,6 +5,7 @@ import { Hero } from "../components/Hero";
 import { PainPoints } from "../components/PainPoints";
 import { FeatureGrid } from "../components/FeatureGrid";
 import { GetStarted } from "../components/GetStarted";
+import { DownloadFoundry } from "../components/DownloadFoundry";
 import { Inspector } from "../components/Inspector";
 import { FAQ } from "../components/FAQ";
 import { Footer } from "../components/Footer";
@@ -17,6 +18,7 @@ import { Footer } from "../components/Footer";
       <PainPoints client:visible />
       <FeatureGrid client:visible />
       <GetStarted client:visible />
+      <DownloadFoundry client:visible />
       <Inspector client:visible />
       <FAQ client:visible />
     </main>