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>

config/default_config.yaml

@@ -68,11 +68,14 @@ web_server:
   port: 8080
   host: "127.0.0.1"
 
-remote_processing:
-  enabled: false  # Enable remote transcription offloading
-  server_url: ""  # WebSocket URL of remote transcription service (e.g., ws://your-server:8765/ws/transcribe)
-  api_key: ""  # API key for authentication
-  fallback_to_local: true  # Fall back to local processing if remote fails
+remote:
+  mode: local  # local | managed | byok
+  server_url: ""  # Proxy server URL for managed mode (e.g., wss://your-proxy.com)
+  auth_token: ""  # JWT stored after login (managed mode)
+  byok_api_key: ""  # Deepgram API key for BYOK mode
+  deepgram_model: nova-2  # Deepgram model to use
+  language: en-US  # Language code
+  fallback_to_local: true  # Fall back to local Whisper if remote fails
 
 updates:
   auto_check: true  # Check for updates on startup