fix(player): show '--:--' for unknown duration instead of '0:00'

Prevents the silly "2:01 / 0:00" display when audio src is being swapped
from preview to full audio and duration hasn't loaded yet.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ui/src/routes/+layout.svelte

@@ -268,6 +268,11 @@
 		}, 2000) as unknown as number;
 	}
 
+	function formatDuration(s: number): string {
+		if (!isFinite(s) || s <= 0) return '--:--';
+		return formatTime(s);
+	}
+
 	function formatTime(s: number): string {
 		if (!isFinite(s) || s < 0) return '0:00';
 		const m = Math.floor(s / 60);
@@ -872,7 +877,7 @@
 					</p>
 				{:else if audioStore.status === 'ready'}
 					<p class="text-xs text-(--color-muted) tabular-nums leading-tight flex items-center gap-1.5">
-						{formatTime(audioStore.currentTime)} / {formatTime(audioStore.duration)}
+						{formatTime(audioStore.currentTime)} / {formatDuration(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}