feat: add file watching for Whisper models and settings

[wilcorrea]

Context

During PR #2 review, CodeRabbit suggested adding notify-based file watching for the Whisper models directory and settings file. This would enable the app to reactively refresh when external changes occur.

Current State

• ✅ The app already uses notify crate for watching markdown files (lib.rs:88-109)
• ✅ model_manager.rs owns the paths and I/O logic for settings and models
• ❌ No auto-refresh when models are added/deleted externally
• ❌ No auto-refresh when settings change externally

Proposed Implementation

Add two watcher helper functions to whisper/model_manager.rs:

1. Watch Models Directory

pub fn watch_models_dir(
    models_dir: &PathBuf, 
    tx: std::sync::mpsc::Sender<notify::Event>
) -> notify::Result<notify::RecommendedWatcher> {
    let mut watcher = notify::recommended_watcher(move |res: Result<Event, notify::Error>| {
        if let Ok(event) = res {
            let _ = tx.send(event);
        }
    })?;
    
    watcher.watch(models_dir, RecursiveMode::Recursive)?;
    Ok(watcher)
}

2. Watch Settings File

pub fn watch_settings_file(
    settings_path: &PathBuf,
    tx: std::sync::mpsc::Sender<notify::Event>
) -> notify::Result<notify::RecommendedWatcher> {
    let mut watcher = notify::recommended_watcher(move |res: Result<Event, notify::Error>| {
        if let Ok(event) = res {
            let _ = tx.send(event);
        }
    })?;
    
    watcher.watch(settings_path, RecursiveMode::NonRecursive)?;
    Ok(watcher)
}

Integration Points

Update lib.rs to:

1. Create watchers in setup() similar to markdown file watching
2. Store watchers in app state (e.g., WhisperWatcherState)
3. Emit events to frontend: whisper:models-changed, whisper:settings-changed

Frontend Reactions

When events are received:

• models-changed: Refresh model list UI (list_models())
• settings-changed: Reload settings, update active model/shortcut

Use Cases

1. Model Management: User downloads model via CLI/script → UI auto-updates
2. Settings Sync: Settings changed by another app instance → current instance reflects changes
3. External Tools: Models managed by external tools → app stays in sync

Optional Enhancements

• Debounce rapid events using notify-debouncer-mini
• Add #[tauri::command] to manually trigger refresh
• Add toast notification when external changes detected

Testing Checklist

• Add model file manually → UI refreshes
• Delete model file → UI updates list
• Edit settings file externally → app reflects changes
• Verify no memory leaks with long-running watchers
• Test on macOS and Linux (different notify backends)

References

• Original PR: feat: add offline voice-to-text with Whisper and UI improvements #2
• CodeRabbit comment: feat: add offline voice-to-text with Whisper and UI improvements #2 (comment)
• Existing watcher implementation: lib.rs:88-109
• notify docs: https://docs.rs/notify/latest/notify/

Notes

This aligns with the project's coding guidelines to use notify for file watching and improves UX by making the app reactive to external changes.