Add Tauri v2 + Svelte 5 frontend and headless Python backend

Scaffold the cross-platform rewrite from PySide6/Qt to Tauri + Svelte,
following the same architecture as voice-to-notes. The Python backend
runs headless as a sidecar, with a FastAPI control API that the Svelte
frontend connects to via REST and WebSocket.

New files:
- backend/app_controller.py: Headless orchestration (extracted from MainWindow)
- backend/api_server.py: FastAPI control endpoints + /ws/control WebSocket
- backend/main_headless.py: Headless entry point for sidecar mode
- src-tauri/: Tauri v2 Rust shell with sidecar and dialog plugins
- src/: Svelte 5 frontend (App, Settings, Controls, TranscriptionDisplay)
- src/lib/stores/: Reactive stores for backend connection, config, transcriptions

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

src-tauri/src/lib.rs

@@ -0,0 +1,9 @@
+#[cfg_attr(mobile, tauri::mobile_entry_point)]
+pub fn run() {
+    tauri::Builder::default()
+        .plugin(tauri_plugin_shell::init())
+        .plugin(tauri_plugin_dialog::init())
+        .plugin(tauri_plugin_process::init())
+        .run(tauri::generate_context!())
+        .expect("error while running tauri application");
+}