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  path superseded by the live transcription session)
- transcribe_pcm, transcribe_pcm_parakeet (PCM commands superseded by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/windows.rs

@@ -27,12 +27,6 @@ pub async fn open_preview_window(_app: tauri::AppHandle) -> Result<(), String> {
     Err(ANDROID_MULTIWINDOW_ERR.into())
 }
 
-#[cfg(target_os = "android")]
-#[tauri::command]
-pub async fn close_preview_window(_app: tauri::AppHandle) -> Result<(), String> {
-    Err(ANDROID_MULTIWINDOW_ERR.into())
-}
-
 #[cfg(target_os = "android")]
 #[tauri::command]
 pub async fn open_viewer_window(_app: tauri::AppHandle) -> Result<(), String> {
@@ -152,17 +146,6 @@ pub async fn open_preview_window(app: tauri::AppHandle) -> Result<(), String> {
     Ok(())
 }
 
-/// Hide the transcription preview window without destroying it so the next
-/// open is instant. Returns Ok even when no preview window exists.
-#[cfg(not(target_os = "android"))]
-#[tauri::command]
-pub async fn close_preview_window(app: tauri::AppHandle) -> Result<(), String> {
-    if let Some(window) = app.get_webview_window("transcription-preview") {
-        window.hide().map_err(|e| e.to_string())?;
-    }
-    Ok(())
-}
-
 /// Open the transcript viewer window.
 #[cfg(not(target_os = "android"))]
 #[tauri::command]