fix(ui): stop audio restarting by removing conditional <audio> mount and normalised src comparison

Two bugs caused the start/stop loop:
1. The <audio> element was wrapped in {#if audioStore.audioUrl}, so whenever
   any reactive state changed (e.g. currentTime ticking), Svelte could destroy
   and recreate the element, firing onpause and then the URL effect restarting
   playback.
2. Comparing audioEl.src !== url is unreliable — browsers normalise the src
   property to a full absolute URL, causing false mismatches every tick.

Fix: make <audio> always present in the DOM (display:none), and track the
loaded URL in a plain local variable (loadedUrl) instead of reading audioEl.src.

ui/src/routes/+layout.svelte

@@ -17,14 +17,20 @@
 	});
 
 	// When audioUrl changes, load the new source.
+	// Use a local variable to track which URL is currently loaded so we never
+	// compare against audioEl.src (browsers normalise it, causing false mismatches).
+	let loadedUrl = '';
 	$effect(() => {
 		if (!audioEl) return;
 		const url = audioStore.audioUrl;
-		if (url && audioEl.src !== url) {
+		if (url && url !== loadedUrl) {
+			loadedUrl = url;
 			audioEl.src = url;
 			audioEl.load();
 			audioEl.playbackRate = audioStore.speed;
 			audioEl.play().catch(() => {});
+		} else if (!url) {
+			loadedUrl = '';
 		}
 	});
 
@@ -107,19 +113,19 @@
 	<title>libnovel</title>
 </svelte:head>
 
-<!-- Hidden persistent audio element — lives outside {#key} so it never unmounts -->
-{#if audioStore.audioUrl}
-	<audio
-		bind:this={audioEl}
-		bind:currentTime={audioStore.currentTime}
-		bind:duration={audioStore.duration}
-		onplay={() => (audioStore.isPlaying = true)}
-		onpause={() => (audioStore.isPlaying = false)}
-		onended={() => (audioStore.isPlaying = false)}
-		preload="metadata"
-		style="display:none"
-	></audio>
-{/if}
+<!-- Persistent audio element — always in the DOM, never conditionally unmounted.
+     Conditional rendering ({#if}) would destroy/recreate it when reactive state
+     changes (e.g. currentTime ticking), triggering onpause and restarting audio. -->
+<audio
+	bind:this={audioEl}
+	bind:currentTime={audioStore.currentTime}
+	bind:duration={audioStore.duration}
+	onplay={() => (audioStore.isPlaying = true)}
+	onpause={() => (audioStore.isPlaying = false)}
+	onended={() => (audioStore.isPlaying = false)}
+	preload="metadata"
+	style="display:none"
+></audio>
 
 <div class="min-h-screen flex flex-col" class:pb-24={audioStore.active}>
 	<header class="border-b border-zinc-700 bg-zinc-900 sticky top-0 z-50">