Make DevTools a toggle in Settings > Developer tab

- DevTools off by default (no more auto-open on launch)
- New "Developer" tab in Settings with a checkbox to toggle devtools
- Toggle takes effect immediately (opens/closes inspector)
- Setting persists: devtools restored on next launch if enabled
- toggle_devtools Tauri command wraps window.open/close_devtools

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/stores/settings.ts

@@ -18,6 +18,7 @@ export interface AppSettings {
   skip_diarization: boolean;
   hf_token: string;
   num_speakers: number | null;
+  devtools_enabled: boolean;
 }
 
 const defaults: AppSettings = {
@@ -37,6 +38,7 @@ const defaults: AppSettings = {
   skip_diarization: false,
   hf_token: '',
   num_speakers: null,
+  devtools_enabled: false,
 };
 
 export const settings = writable<AppSettings>({ ...defaults });