refactor(profile): visual voice picker, playback toggles, danger zone

- Replace voice <select> with a two-column card grid grouped by engine
  (Kokoro GPU / Pocket TTS CPU / Cloudflare AI); each card has a per-voice
  sample play/pause button matching AudioPlayer behaviour
- Add Announce chapter and Audio mode (Stream/Generate) toggles to a
  unified Playback row in Preferences; Audio mode toggle disabled for
  CF AI voices
- Remove duplicate PUT /api/settings from the profile page; all writes
  go directly into audioStore / theme context and the layout's single
  debounced effect persists them
- Add Danger Zone section: collapsible, requires typing username to
  unlock Delete account button; calls DELETE /api/profile
- Add deleteUserAccount() to pocketbase.ts: purges user_settings,
  user_library, progress, comment_votes, book_ratings,
  user_subscriptions, notifications, user_sessions then the
  app_users record
- Add DELETE /api/profile server route (auth-guarded)

ui/src/routes/api/profile/+server.ts

@@ -0,0 +1,26 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { deleteUserAccount } from '$lib/server/pocketbase';
+import { log } from '$lib/server/logger';
+
+/**
+ * DELETE /api/profile
+ *
+ * Permanently deletes the authenticated user's account and all associated data:
+ * settings, library, progress, votes, ratings, sessions, notifications.
+ *
+ * The app_users record is removed last. The caller should immediately log the
+ * user out (submit the logout form) to clear the session cookie.
+ */
+export const DELETE: RequestHandler = async ({ locals }) => {
+	if (!locals.user) error(401, 'Not authenticated');
+
+	try {
+		await deleteUserAccount(locals.user.id, locals.sessionId);
+	} catch (e) {
+		log.error('profile', 'DELETE /api/profile failed', { userId: locals.user.id, err: String(e) });
+		error(500, { message: 'Failed to delete account. Please try again or contact support.' });
+	}
+
+	return json({ ok: true });
+};