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/commands/transcribe.rs

@@ -40,6 +40,35 @@ pub fn transcribe_file(
     Ok(response.payload)
 }
 
+/// Download and validate the diarization model via the Python sidecar.
+#[tauri::command]
+pub fn download_diarize_model(
+    hf_token: String,
+) -> Result<Value, String> {
+    let manager = sidecar();
+    manager.ensure_running()?;
+
+    let request_id = uuid::Uuid::new_v4().to_string();
+    let msg = IPCMessage::new(
+        &request_id,
+        "diarize.download",
+        json!({
+            "hf_token": hf_token,
+        }),
+    );
+
+    let response = manager.send_and_receive(&msg)?;
+
+    if response.msg_type == "error" {
+        return Ok(json!({
+            "ok": false,
+            "error": response.payload.get("message").and_then(|v| v.as_str()).unwrap_or("unknown"),
+        }));
+    }
+
+    Ok(json!({ "ok": true }))
+}
+
 /// Run the full transcription + diarization pipeline via the Python sidecar.
 #[tauri::command]
 pub fn run_pipeline(