Phase 6: Add Deepgram remote transcription (managed + BYOK modes)

New files:
- client/deepgram_transcription.py — DeepgramTranscriptionEngine with
  managed mode (proxy) and BYOK mode (direct Deepgram). Sends raw binary
  PCM audio over WebSocket, handles both proxy and Deepgram response formats.

Modified files:
- config/default_config.yaml — Replace remote_processing with new remote
  section (mode, server_url, auth_token, byok_api_key, deepgram_model, language)
- client/config.py — Add migration from old remote_processing config
- gui/settings_dialog_qt.py — Replace Remote Processing group with
  Transcription Mode section (Local/Managed/BYOK radio buttons, login/register
  dialogs, balance display, model selector)
- gui/main_window_qt.py — Select engine based on remote.mode config,
  add error and credits_low handlers

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

client/config.py

@@ -48,6 +48,25 @@ class Config:
             # Save the default configuration
             self.save()
 
+        # Migrate remote_processing -> remote
+        self._migrate_remote_config()
+
+    def _migrate_remote_config(self):
+        """Migrate old remote_processing config to new remote config."""
+        if 'remote_processing' in self.config and 'remote' not in self.config:
+            old = self.config['remote_processing']
+            self.config['remote'] = {
+                'mode': 'managed' if old.get('enabled', False) else 'local',
+                'server_url': old.get('server_url', ''),
+                'auth_token': '',
+                'byok_api_key': old.get('api_key', ''),
+                'deepgram_model': 'nova-2',
+                'language': 'en-US',
+                'fallback_to_local': old.get('fallback_to_local', True),
+            }
+            del self.config['remote_processing']
+            self.save()
+
     def save(self) -> None:
         """Save current configuration to file."""
         with open(self.config_path, 'w') as f: