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>

src-tauri/src/lib.rs

@@ -12,7 +12,7 @@ use commands::export::export_transcript;
 use commands::project::{create_project, get_project, list_projects};
 use commands::settings::{load_settings, save_settings};
 use commands::system::{get_data_dir, llama_list_models, llama_start, llama_status, llama_stop};
-use commands::transcribe::{run_pipeline, transcribe_file};
+use commands::transcribe::{download_diarize_model, run_pipeline, transcribe_file};
 use state::AppState;
 
 #[cfg_attr(mobile, tauri::mobile_entry_point)]
@@ -36,6 +36,7 @@ pub fn run() {
             list_projects,
             transcribe_file,
             run_pipeline,
+            download_diarize_model,
             export_transcript,
             ai_chat,
             ai_list_providers,