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/capabilities/default.json

@@ -0,0 +1,30 @@
+{
+  "identifier": "default",
+  "description": "Default capability for Foundry desktop",
+  "windows": ["main"],
+  "permissions": [
+    "core:default",
+    "core:window:allow-start-dragging",
+    "shell:allow-open",
+    {
+      "identifier": "shell:allow-execute",
+      "allow": [
+        {
+          "name": "sidecars/foundry-backend",
+          "sidecar": true,
+          "args": true
+        }
+      ]
+    },
+    {
+      "identifier": "shell:allow-spawn",
+      "allow": [
+        {
+          "name": "sidecars/foundry-backend",
+          "sidecar": true,
+          "args": true
+        }
+      ]
+    }
+  ]
+}