Fix progress feedback, diarization fallback, and dropdown readability

- Stream pipeline progress to frontend via Tauri events so the progress
  overlay updates in real time during transcription/diarization
- Gracefully fall back to transcription-only when diarization fails
  (e.g. pyannote not installed) instead of erroring the whole pipeline
- Add color-scheme: dark to fix native select/option elements rendering
  with unreadable white backgrounds

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

src-tauri/src/sidecar/mod.rs

@@ -115,8 +115,17 @@ impl SidecarManager {
     }
 
     /// Send a message to the sidecar and read the response.
-    /// This is a blocking call.
+    /// This is a blocking call. Progress messages are skipped.
     pub fn send_and_receive(&self, msg: &IPCMessage) -> Result<IPCMessage, String> {
+        self.send_and_receive_with_progress(msg, |_| {})
+    }
+
+    /// Send a message and read the response, calling on_progress for each progress message.
+    pub fn send_and_receive_with_progress(
+        &self,
+        msg: &IPCMessage,
+        on_progress: impl Fn(&IPCMessage),
+    ) -> Result<IPCMessage, String> {
         // Write to stdin
         {
             let mut stdin_guard = self.stdin.lock().map_err(|e| e.to_string())?;
@@ -154,10 +163,11 @@ impl SidecarManager {
                     let response: IPCMessage = serde_json::from_str(trimmed)
                         .map_err(|e| format!("Parse error: {e}"))?;
 
-                    // Skip progress messages, return the final result/error
-                    if response.msg_type != "progress" {
-                        return Ok(response);
+                    if response.msg_type == "progress" {
+                        on_progress(&response);
+                        continue;
                     }
+                    return Ok(response);
                 }
             } else {
                 Err("Sidecar stdout not available".to_string())