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/+layout.server.ts

@@ -17,7 +17,7 @@ export const load: LayoutServerLoad = async ({ locals, url, cookies }) => {
 		redirect(302, `/login`);
 	}
 
-	let settings = { autoNext: false, voice: 'af_bella', speed: 1.0, theme: 'amber', locale: 'en', fontFamily: 'system', fontSize: 1.0 };
+	let settings = { autoNext: false, voice: 'af_bella', speed: 1.0, theme: 'amber', locale: 'en', fontFamily: 'system', fontSize: 1.0, announceChapter: false };
 	try {
 		const row = await getSettings(locals.sessionId, locals.user?.id);
 		if (row) {
@@ -28,7 +28,8 @@ export const load: LayoutServerLoad = async ({ locals, url, cookies }) => {
 				theme: row.theme ?? 'amber',
 				locale: row.locale ?? 'en',
 				fontFamily: row.font_family ?? 'system',
-				fontSize: row.font_size || 1.0
+				fontSize: row.font_size || 1.0,
+				announceChapter: row.announce_chapter ?? false
 			};
 		}
 	} catch (e) {