Fix dev mode sidecar launch and engine reload on mode change

1. Dev mode: use `uv run python` instead of bare `python` to ensure
   the project venv is used. Also use CARGO_MANIFEST_DIR to find the
   project root reliably.

2. Engine reload: changing remote.mode (local/managed/byok) now
   triggers a full engine reload. Previously only model and device
   changes triggered reload, so switching to Deepgram had no effect
   until the app was restarted.

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

backend/app_controller.py

@@ -577,12 +577,18 @@ class AppController:
             if self.config.get('server_sync.enabled', False):
                 self._start_server_sync()
 
-        # Check if model/device changed
+        # Check if model/device/remote mode changed -- any of these require
+        # a full engine reload since they change which engine class is used
         new_model = self.config.get('transcription.model', 'base.en')
         new_device = self.config.get('transcription.device', 'auto')
+        new_remote_mode = self.config.get('remote.mode', 'local')
+        current_remote_mode = 'local'
+        if self.transcription_engine:
+            current_remote_mode = getattr(self.transcription_engine, 'mode', 'local')
         engine_reload_needed = (
             self.current_model_size != new_model
             or self.current_device_config != new_device
+            or current_remote_mode != new_remote_mode
         )
 
         if engine_reload_needed: