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/scripts/build-frontend.ts

@@ -0,0 +1,42 @@
+import { execSync } from "node:child_process";
+import { cpSync, readFileSync, writeFileSync, rmSync, existsSync } from "node:fs";
+import { resolve, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const desktopRoot = resolve(__dirname, "..");
+const repoRoot = resolve(desktopRoot, "../../..");
+const frontendDist = resolve(desktopRoot, "../frontend/dist");
+const destDir = resolve(desktopRoot, "frontend-dist");
+
+function run(cmd: string, opts?: { cwd?: string; env?: NodeJS.ProcessEnv }) {
+  console.log(`> ${cmd}`);
+  execSync(cmd, {
+    stdio: "inherit",
+    cwd: opts?.cwd ?? repoRoot,
+    env: { ...process.env, ...opts?.env },
+  });
+}
+
+// Step 1: Build the frontend with the desktop-specific backend endpoint
+console.log("\n=== Building frontend for desktop ===\n");
+run("pnpm --filter @sandbox-agent/foundry-frontend build", {
+  env: {
+    VITE_HF_BACKEND_ENDPOINT: "http://127.0.0.1:7741/api/rivet",
+  },
+});
+
+// Step 2: Copy dist to frontend-dist/
+console.log("\n=== Copying frontend build output ===\n");
+if (existsSync(destDir)) {
+  rmSync(destDir, { recursive: true });
+}
+cpSync(frontendDist, destDir, { recursive: true });
+
+// Step 3: Strip react-scan script from index.html (it loads unconditionally)
+const indexPath = resolve(destDir, "index.html");
+let html = readFileSync(indexPath, "utf-8");
+html = html.replace(/<script\s+src="https:\/\/unpkg\.com\/react-scan\/dist\/auto\.global\.js"[^>]*><\/script>\s*/g, "");
+writeFileSync(indexPath, html);
+
+console.log("\n=== Frontend build complete ===\n");