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/commands/transcribe.rs

@@ -1,4 +1,5 @@
 use serde_json::{json, Value};
+use tauri::{AppHandle, Emitter};
 
 use crate::sidecar::messages::IPCMessage;
 use crate::sidecar::sidecar;
@@ -42,6 +43,7 @@ pub fn transcribe_file(
 /// Run the full transcription + diarization pipeline via the Python sidecar.
 #[tauri::command]
 pub fn run_pipeline(
+    app: AppHandle,
     file_path: String,
     model: Option<String>,
     device: Option<String>,
@@ -71,7 +73,9 @@ pub fn run_pipeline(
         }),
     );
 
-    let response = manager.send_and_receive(&msg)?;
+    let response = manager.send_and_receive_with_progress(&msg, |progress| {
+        let _ = app.emit("pipeline-progress", &progress.payload);
+    })?;
 
     if response.msg_type == "error" {
         return Err(format!(