Add Test & Download button for diarization model, clickable links

- Add diarize.download IPC handler that downloads the pyannote model
  and returns user-friendly error messages (missing license, bad token)
- Add download_diarize_model Tauri command
- Add "Test & Download Model" button in Speakers settings tab
- Update instructions to list both required model licenses
  (speaker-diarization-3.1 AND segmentation-3.0)
- Make all HuggingFace URLs clickable (opens in system browser)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

python/voice_to_notes/main.py

@@ -15,6 +15,7 @@ from voice_to_notes.ipc.handlers import (  # noqa: E402
     HandlerRegistry,
     hardware_detect_handler,
     make_ai_chat_handler,
+    make_diarize_download_handler,
     make_diarize_handler,
     make_export_handler,
     make_pipeline_handler,
@@ -32,6 +33,7 @@ def create_registry() -> HandlerRegistry:
     registry.register("transcribe.start", make_transcribe_handler())
     registry.register("hardware.detect", hardware_detect_handler)
     registry.register("diarize.start", make_diarize_handler())
+    registry.register("diarize.download", make_diarize_download_handler())
     registry.register("pipeline.start", make_pipeline_handler())
     registry.register("export.start", make_export_handler())
     registry.register("ai.chat", make_ai_chat_handler())