feat: Tauri desktop GUI app with call engine

- New desktop/ directory with Tauri v2 + Vite + TypeScript
- Rust backend: CallEngine wrapping wzp-client audio + transport
- Web frontend: connect screen, in-call screen with participants,
  mic/speaker mute, keyboard shortcuts (m/s/q)
- Dark theme UI, settings persistence via localStorage
- Platform-aware --os-aec: warns on Windows/Linux (not yet implemented)
- Workspace updated to include desktop/src-tauri

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktop/vite.config.ts

@@ -0,0 +1,15 @@
+import { defineConfig } from "vite";
+
+export default defineConfig({
+  clearScreen: false,
+  server: {
+    port: 1420,
+    strictPort: true,
+  },
+  envPrefix: ["VITE_", "TAURI_"],
+  build: {
+    target: "esnext",
+    minify: !process.env.TAURI_DEBUG ? "esbuild" : false,
+    sourcemap: !!process.env.TAURI_DEBUG,
+  },
+});