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.svelte

@@ -157,6 +157,7 @@
 				audioStore.voice = data.settings.voice;
 				audioStore.speed = data.settings.speed;
 				audioStore.announceChapter = data.settings.announceChapter ?? false;
+				audioStore.audioMode = (data.settings.audioMode === 'generate' ? 'generate' : 'stream');
 			}
 			// Always sync theme + font (profile page calls invalidateAll after saving)
 			currentTheme = data.settings.theme ?? 'amber';
@@ -179,6 +180,7 @@
 		const fontFamily = currentFontFamily;
 		const fontSize = currentFontSize;
 		const announceChapter = audioStore.announceChapter;
+		const audioMode = audioStore.audioMode;
 
 		// Skip saving until settings have been applied from the server AND
 		// at least one user-driven change has occurred after that.
@@ -189,7 +191,7 @@
 			fetch('/api/settings', {
 				method: 'PUT',
 				headers: { 'Content-Type': 'application/json' },
-				body: JSON.stringify({ autoNext, voice, speed, theme, fontFamily, fontSize, announceChapter })
+				body: JSON.stringify({ autoNext, voice, speed, theme, fontFamily, fontSize, announceChapter, audioMode })
 			}).catch(() => {});
 		}, 800) as unknown as number;
 	});