chore: remove 11 orphan Tauri commands and downstream dead code

Sweep of registered-but-never-invoked commands surfaced by an audit
against the frontend invoke() call sites. Each was confirmed dead via
grep across src-tauri, src, and crates: no caller anywhere.

Removed commands:
- check_model, count_transcripts_command, get_profile_cmd,
  install_update, list_feedback_examples_cmd (utility/CRUD shapes
  never wired)
- save_audio, start_native_capture, stop_native_capture (native-capture
  path superseded by the live transcription session)
- transcribe_pcm, transcribe_pcm_parakeet (PCM commands superseded by
  live session; no frontend caller)
- close_preview_window (preview window is hidden via the
  core:window:allow-hide capability, not the command)

Cascade in audio.rs (~430 lines removed):
- CaptureWorker, NativeCaptureState struct + impl, stop_worker,
  append_recorded_chunk, MAX_NATIVE_CAPTURE_RETURN_SAMPLES,
  persist_audio_samples
- The two cfg(test) tests that exercised stop_worker (the
  recording_filename tests stay, supporting resolve_recording_path
  which the live session uses)

Cascade elsewhere:
- FeedbackDto struct and its From<FeedbackRow> impl
- Stale storage imports in feedback.rs, profiles.rs, transcripts.rs
- tauri::Emitter import in transcription.rs
- app.manage(NativeCaptureState::new()) in lib.rs setup

generate_handler entries removed for all 11 commands. cargo check passes
cleanly with zero warnings; no tests reference any deleted symbols.

Net: 709 deletions, 17 insertions across 9 files.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/models.rs

@@ -529,12 +529,6 @@ pub async fn download_model(
     Ok(format!("Model {} downloaded", size))
 }
 
-#[tauri::command]
-pub fn check_model(size: String) -> Result<bool, String> {
-    let id = whisper_model_id(&size);
-    Ok(model_manager::is_downloaded(&id))
-}
-
 #[tauri::command]
 pub fn list_models() -> Result<Vec<String>, String> {
     let ids = model_manager::list_downloaded();