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>

.claude/settings.local.json

@@ -0,0 +1,9 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(python3:*)",
+      "Bash(node --check:*)",
+      "Bash(ls:*)"
+    ]
+  }
+}