Add ability to change transcription engine from Settings

New features:
- Settings > Transcription Engine > "Change Transcription Engine"
  button stops the sidecar, deletes downloaded files, and reloads
  the app to show the engine selection screen
- Improved SidecarSetup descriptions with detailed explanations
  of each variant and "Recommended" tag on Cloud (Deepgram)
- Cloud option listed first as the recommended choice
- New reset_sidecar Tauri command that cleans up sidecar files

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -68,6 +68,7 @@ pub fn run() {
             sidecar::get_sidecar_port,
             sidecar::start_sidecar,
             sidecar::stop_sidecar,
+            sidecar::reset_sidecar,
             write_log,
         ])
         .run(tauri::generate_context!())