feat(player): CF AI preview/swap + fix PB token expiry + local build time

- Backend: add GET /api/audio-preview/{slug}/{n} — generates first ~1800-char
  chunk via CF AI so playback starts immediately; full chapter cached in MinIO
- Frontend: replace CF AI spinner with preview blob URL + background swap to
  full presigned URL when runner finishes, preserving currentTime
- AudioPlayer: isPreview state + 'preview' badge in mini-bar during swap
- pocketbase.ts: fix 403 on stale token — reduce TTL to 50 min + retry once
  on 401/403 with forced re-auth (was cached 12 h, PB tokens expire in 1 h)
- Footer build time now rendered in user's local timezone via toLocaleString()

ui/src/routes/+layout.svelte

@@ -37,6 +37,10 @@
 	let activeChapterEl = $state<HTMLElement | null>(null);
 	let listeningModeOpen = $state(false);
 
+	// Build time formatted in the user's local timezone (populated on mount so
+	// SSR and CSR don't produce a mismatch — SSR renders nothing, hydration fills it in).
+	let buildTimeLocal = $state('');
+
 	function setIfActive(node: HTMLElement, isActive: boolean) {
 		if (isActive) activeChapterEl = node;
 		return {
@@ -51,6 +55,12 @@
 		}
 	});
 
+	$effect(() => {
+		if (env.PUBLIC_BUILD_TIME && env.PUBLIC_BUILD_TIME !== 'unknown') {
+			buildTimeLocal = new Date(env.PUBLIC_BUILD_TIME).toLocaleString();
+		}
+	});
+
 	// The single <audio> element that persists across navigations.
 	// AudioPlayer components in chapter pages control it via audioStore.
 	let audioEl = $state<HTMLAudioElement | null>(null);
@@ -750,10 +760,9 @@
 		</div>
 		<!-- Build version / commit SHA / build time -->
 		{#snippet buildTime()}
-			{#if env.PUBLIC_BUILD_TIME && env.PUBLIC_BUILD_TIME !== 'unknown'}
-				{@const d = new Date(env.PUBLIC_BUILD_TIME)}
+			{#if buildTimeLocal}
 				<span class="text-(--color-muted)" title="Build time">
-					· {d.toUTCString().replace(' GMT', ' UTC').replace(/:\d\d /, ' ')}
+					· {buildTimeLocal}
 				</span>
 			{/if}
 		{/snippet}
@@ -862,8 +871,11 @@
 						{m.player_generating({ percent: String(Math.round(audioStore.progress)) })}
 					</p>
 				{:else if audioStore.status === 'ready'}
-					<p class="text-xs text-(--color-muted) tabular-nums leading-tight">
+					<p class="text-xs text-(--color-muted) tabular-nums leading-tight flex items-center gap-1.5">
 						{formatTime(audioStore.currentTime)} / {formatTime(audioStore.duration)}
+						{#if audioStore.isPreview}
+							<span class="px-1 py-0.5 rounded text-[10px] font-medium bg-(--color-brand)/15 text-(--color-brand) leading-none">preview</span>
+						{/if}
 					</p>
 				{:else if audioStore.status === 'loading'}
 					<p class="text-xs text-(--color-muted) leading-tight">{m.player_loading()}</p>