feat: add chapter announcing setting to audio player

When enabled, the Web Speech API speaks the upcoming chapter number and
title (e.g. 'Chapter 12 — The Final Battle') between auto-next chapters,
giving an audible cue before the next narration begins.

- AudioStore.announceChapter ( boolean, default false)
- PBUserSettings.announce_chapter persisted to PocketBase
- GET/PUT /api/settings includes announceChapter field
- +layout.server.ts loads + defaults the field
- +layout.svelte applies on load, saves in debounced PUT, and fires
  SpeechSynthesisUtterance in onended before navigating (falls back to
  immediate navigation if speechSynthesis is unavailable)
- ListeningMode: 'Announce' pill added to the Speed · Auto · Sleep row

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

@@ -5,7 +5,7 @@ import { log } from '$lib/server/logger';
 
 /**
  * GET /api/settings
- * Returns the current user's settings (auto_next, voice, speed, theme, locale, fontFamily, fontSize).
+ * Returns the current user's settings (auto_next, voice, speed, theme, locale, fontFamily, fontSize, announceChapter).
  * Returns defaults if no settings record exists yet.
  */
 export const GET: RequestHandler = async ({ locals }) => {
@@ -18,7 +18,8 @@ export const GET: RequestHandler = async ({ locals }) => {
 			theme: settings?.theme ?? 'amber',
 			locale: settings?.locale ?? 'en',
 			fontFamily: settings?.font_family ?? 'system',
-			fontSize: settings?.font_size || 1.0
+			fontSize: settings?.font_size || 1.0,
+			announceChapter: settings?.announce_chapter ?? false
 		});
 	} catch (e) {
 		log.error('settings', 'GET failed', { err: String(e) });
@@ -28,7 +29,7 @@ export const GET: RequestHandler = async ({ locals }) => {
 
 /**
  * PUT /api/settings
- * Body: { autoNext: boolean, voice: string, speed: number, theme?: string, locale?: string, fontFamily?: string, fontSize?: number }
+ * Body: { autoNext: boolean, voice: string, speed: number, theme?: string, locale?: string, fontFamily?: string, fontSize?: number, announceChapter?: boolean }
  * Saves user preferences.
  */
 export const PUT: RequestHandler = async ({ request, locals }) => {
@@ -67,6 +68,11 @@ export const PUT: RequestHandler = async ({ request, locals }) => {
 		error(400, `Invalid fontSize — must be one of: ${validFontSizes.join(', ')}`);
 	}
 
+	// announceChapter is optional boolean
+	if (body.announceChapter !== undefined && typeof body.announceChapter !== 'boolean') {
+		error(400, 'Invalid announceChapter — must be boolean');
+	}
+
 	try {
 		await saveSettings(locals.sessionId, body, locals.user?.id);
 	} catch (e) {