feat: stream/generate audio mode toggle

Add a user-selectable playback mode stored in user_settings:
- 'stream' (default): /api/audio-stream starts playing within seconds,
  saves to MinIO concurrently — low latency
- 'generate': queue runner task, poll until full audio is ready in
  MinIO, then play via presigned URL — legacy behaviour

UI toggles in two places:
- AudioPlayer idle pill: compact '· Stream / · Generate' inline next
  to voice name and estimated duration
- ListeningMode controls row: pill alongside Auto, Announce, Sleep;
  disabled and grayed out for CF AI voices (batch-only, no streaming)

startPlayback() now branches on audioStore.audioMode for non-CF AI
voices; generate mode uses the same runner task + progress bar flow
as CF AI but without the preview clip.

PocketBase: audio_mode text field added to user_settings on
pb.libnovel.cc (live) and in pb-init-v3.sh (create block +
add_field migration line).

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, announceChapter: false };
+	let settings = { autoNext: false, voice: 'af_bella', speed: 1.0, theme: 'amber', locale: 'en', fontFamily: 'system', fontSize: 1.0, announceChapter: false, audioMode: 'stream' };
 	try {
 		const row = await getSettings(locals.sessionId, locals.user?.id);
 		if (row) {
@@ -29,7 +29,8 @@ export const load: LayoutServerLoad = async ({ locals, url, cookies }) => {
 				locale: row.locale ?? 'en',
 				fontFamily: row.font_family ?? 'system',
 				fontSize: row.font_size || 1.0,
-				announceChapter: row.announce_chapter ?? false
+				announceChapter: row.announce_chapter ?? false,
+				audioMode: row.audio_mode ?? 'stream'
 			};
 		}
 	} catch (e) {