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>

Cargo.toml

@@ -10,6 +10,7 @@ members = [
     "crates/wzp-client",
     "crates/wzp-web",
     "crates/wzp-android",
+    "desktop/src-tauri",
 ]
 
 [workspace.package]