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>

backend/main_headless.py

@@ -0,0 +1,126 @@
+#!/usr/bin/env python3
+"""Headless entry point for the Local Transcription backend.
+
+Runs the transcription engine + API server without any GUI (no PySide6).
+Designed to be launched as a Tauri sidecar or run standalone for development.
+
+Usage:
+    python -m backend.main_headless [--port PORT] [--host HOST]
+
+The backend prints the actual port to stdout as JSON on startup:
+    {"event": "ready", "port": 8080}
+
+This allows the Tauri shell to discover which port the backend bound to.
+"""
+
+import argparse
+import json
+import multiprocessing
+import os
+import signal
+import sys
+from pathlib import Path
+
+# Must be called before anything else for PyInstaller compatibility
+multiprocessing.freeze_support()
+
+if __name__ == "__main__":
+    try:
+        multiprocessing.set_start_method('spawn', force=True)
+    except RuntimeError:
+        pass
+
+# Add project root to path
+project_root = Path(__file__).resolve().parent.parent
+sys.path.insert(0, str(project_root))
+os.chdir(project_root)
+
+from client.instance_lock import InstanceLock
+
+
+def main():
+    parser = argparse.ArgumentParser(description="Local Transcription headless backend")
+    parser.add_argument("--host", default="127.0.0.1", help="API server host (default: 127.0.0.1)")
+    parser.add_argument("--port", type=int, default=8080, help="API server port (default: 8080)")
+    args = parser.parse_args()
+
+    instance_lock = InstanceLock()
+    if not instance_lock.acquire():
+        print(json.dumps({"event": "error", "message": "Another instance is already running"}),
+              flush=True)
+        sys.exit(1)
+
+    def handle_shutdown(signum, frame):
+        print(json.dumps({"event": "shutdown"}), flush=True)
+        if controller:
+            controller.shutdown()
+        instance_lock.release()
+        sys.exit(0)
+
+    signal.signal(signal.SIGTERM, handle_shutdown)
+    signal.signal(signal.SIGINT, handle_shutdown)
+
+    controller = None
+
+    try:
+        from backend.app_controller import AppController
+        from backend.api_server import APIServer
+
+        # Override web server port from CLI arg
+        from client.config import Config
+        config = Config()
+        config.set('web_server.host', args.host)
+        config.set('web_server.port', args.port)
+
+        # Create controller and initialize
+        controller = AppController(config=config)
+
+        # Wire a state callback that prints the ready event
+        def on_state_changed(state, message):
+            event = {"event": "state", "state": state, "message": message}
+            print(json.dumps(event), flush=True)
+
+        controller.on_state_changed = on_state_changed
+
+        # Initialize engine + web server
+        controller.initialize()
+
+        # Create API server wrapping the controller
+        api_server = APIServer(controller)
+
+        # Determine actual port (web server may have shifted if port was in use)
+        actual_port = controller.actual_web_port or args.port
+
+        # Print ready event so Tauri can discover the port
+        print(json.dumps({"event": "ready", "port": actual_port}), flush=True)
+
+        # Run the API server (blocks)
+        import uvicorn
+        import logging
+
+        logging.getLogger("uvicorn").setLevel(logging.ERROR)
+        logging.getLogger("uvicorn.access").setLevel(logging.ERROR)
+
+        uvicorn.run(
+            api_server.app,
+            host=args.host,
+            port=actual_port + 1,  # API on port+1, OBS display on the main port
+            log_level="error",
+            access_log=False,
+        )
+
+    except KeyboardInterrupt:
+        print(json.dumps({"event": "shutdown", "reason": "keyboard_interrupt"}), flush=True)
+    except Exception as e:
+        print(json.dumps({"event": "error", "message": str(e)}), flush=True)
+        import traceback
+        traceback.print_exc()
+        sys.exit(1)
+    finally:
+        if controller:
+            controller.shutdown()
+        instance_lock.release()
+
+
+if __name__ == "__main__":
+    main()