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/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, announceChapter).
+ * Returns the current user's settings (auto_next, voice, speed, theme, locale, fontFamily, fontSize, announceChapter, audioMode).
  * Returns defaults if no settings record exists yet.
  */
 export const GET: RequestHandler = async ({ locals }) => {
@@ -19,7 +19,8 @@ export const GET: RequestHandler = async ({ locals }) => {
 			locale: settings?.locale ?? 'en',
 			fontFamily: settings?.font_family ?? 'system',
 			fontSize: settings?.font_size || 1.0,
-			announceChapter: settings?.announce_chapter ?? false
+			announceChapter: settings?.announce_chapter ?? false,
+			audioMode: settings?.audio_mode ?? 'stream'
 		});
 	} catch (e) {
 		log.error('settings', 'GET failed', { err: String(e) });
@@ -29,7 +30,7 @@ export const GET: RequestHandler = async ({ locals }) => {
 
 /**
  * PUT /api/settings
- * Body: { autoNext: boolean, voice: string, speed: number, theme?: string, locale?: string, fontFamily?: string, fontSize?: number, announceChapter?: boolean }
+ * Body: { autoNext: boolean, voice: string, speed: number, theme?: string, locale?: string, fontFamily?: string, fontSize?: number, announceChapter?: boolean, audioMode?: string }
  * Saves user preferences.
  */
 export const PUT: RequestHandler = async ({ request, locals }) => {
@@ -73,6 +74,12 @@ export const PUT: RequestHandler = async ({ request, locals }) => {
 		error(400, 'Invalid announceChapter — must be boolean');
 	}
 
+	// audioMode is optional — if provided it must be a known value
+	const validAudioModes = ['stream', 'generate'];
+	if (body.audioMode !== undefined && !validAudioModes.includes(body.audioMode)) {
+		error(400, `Invalid audioMode — must be one of: ${validAudioModes.join(', ')}`);
+	}
+
 	try {
 		await saveSettings(locals.sessionId, body, locals.user?.id);
 	} catch (e) {