Speed up transcription with a pluggable provider backend

Transcription:
- Provider-pluggable backend: each model routes to OpenAI (multipart
  upload) or OpenRouter (JSON + base64 body). Add OpenRouter's
  whisper-large-v3-turbo (Groq-served, ~216x real-time) to the tray
  menu for far lower latency than whisper-1.
- Load API keys env -> keyring -> file for both providers via a shared
  helper.
- Encode the WAV in memory and reuse a warm requests.Session (no temp file).

Other improvements:
- blurt-toggle signals the running app directly via SIGUSR1 instead of
  paying ~0.5s to import the full stack through "uv run blurt toggle".
- Remove the redundant dead blurt shell script.
- Suppress the benign Gdk-CRITICAL warning from pystray's GtkStatusIcon.
- gitignore openrouter_key.txt.

Transcription code is platform-agnostic; the Windows/Linux platform layer
is unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -7,5 +7,6 @@ __pycache__/
 
 # Blurt runtime config / secrets (written by blurt.py at runtime)
 api_key.txt
+openrouter_key.txt
 model.txt
 language.txt