feat(capture): gate global hotkey on dictation readiness checklist

Stops the "stuck pill" failure where pressing the chord with missing
STT/LLM models triggers a recording that has nowhere to land. The
hotkey now stays disarmed until every gate (models downloaded, Input
Monitoring + Accessibility granted) is green; the empty-state checklist
in CapturesTab surfaces each unmet gate with a one-click action and
auto-arms the chord once everything turns green.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

backend/backends/__init__.py

@@ -480,6 +480,11 @@ def get_llm_model_configs() -> list[ModelConfig]:
     return _get_qwen_llm_configs()
 
 
+def get_stt_model_configs() -> list[ModelConfig]:
+    """Return only STT (Whisper) model configs."""
+    return _get_whisper_configs()
+
+
 # Lookup helpers — these replace the if/elif chains in main.py