feat(ui): replace two-button audio UI with single smart play button and pseudo progress bar

- Single 'Play narration' button: checks presign first, plays immediately if audio exists, otherwise triggers generation
- During generation shows an animated pseudo progress bar: slow start (4%/s) → accelerates (12%/s at 30%) → slows near 80% (4%/s) → crawls to 99% (0.3%/s)
- When generation completes, bar jumps to 100% then transitions to audio player
- Auto-plays audio after generation completes

ui/src/lib/components/AudioPlayer.svelte

@@ -1,10 +1,12 @@
 <script lang="ts">
 	/**
-	 * AudioPlayer — fetches a presigned MinIO URL for the chapter audio
-	 * and renders a native <audio> element with custom controls.
+	 * AudioPlayer — single-button audio player.
 	 *
-	 * The audio is generated server-side by Kokoro and cached.
-	 * If no audio exists yet, the user can trigger generation via the scraper API.
+	 * On click:
+	 *   1. Try GET /api/presign/audio — if 200, load and play.
+	 *   2. If 404, POST /api/audio/:slug/:n to generate. While generating, show
+	 *      a pseudo progress bar that starts slow, accelerates, then slows near
+	 *      80 %. When generation resolves, the bar jumps to 100 % and audio plays.
 	 */
 
 	interface Props {
@@ -16,7 +18,7 @@
 
 	let { slug, chapter, voice = 'af_bella', speed = 1.0 }: Props = $props();
 
-	type Status = 'idle' | 'loading' | 'ready' | 'error' | 'generating';
+	type Status = 'idle' | 'loading' | 'generating' | 'ready' | 'error';
 
 	let status = $state<Status>('idle');
 	let audioUrl = $state('');
@@ -26,46 +28,128 @@
 	let duration = $state(0);
 	let isPlaying = $state(false);
 
-	async function loadAudio() {
-		status = 'loading';
-		errorMsg = '';
-		try {
-			const params = new URLSearchParams({
-				slug,
-				n: String(chapter),
-				voice,
-				speed: String(speed)
-			});
-			const res = await fetch(`/api/presign/audio?${params.toString()}`);
-			if (res.status === 404) {
-				status = 'idle';
-				return;
+	// Pseudo progress bar — value 0-100.
+	let progress = $state(0);
+	let progressRafId = 0;
+
+	// ── Pseudo progress animation ────────────────────────────────────────────
+	// Eases in to ~80 % slowly, stalls, then can be forced to 100 % externally.
+	function startProgress() {
+		progress = 0;
+		let last = performance.now();
+
+		function tick(now: number) {
+			const dt = (now - last) / 1000; // seconds
+			last = now;
+
+			// Speed profile:
+			//   0 – 30 %  → 4 %/s (slow start)
+			//  30 – 60 %  → 12 %/s (ramp up)
+			//  60 – 80 %  → 4 %/s (slow down)
+			//  80 – 99 %  → 0.3 %/s (crawl, never quite reaches 100)
+			let rate: number;
+			if (progress < 30) {
+				rate = 4;
+			} else if (progress < 60) {
+				rate = 12;
+			} else if (progress < 80) {
+				rate = 4;
+			} else {
+				rate = 0.3;
 			}
-			if (!res.ok) throw new Error(`HTTP ${res.status}`);
-			const data = (await res.json()) as { url: string };
-			audioUrl = data.url;
-			status = 'ready';
-		} catch (e) {
-			status = 'error';
-			errorMsg = String(e);
+
+			progress = Math.min(progress + rate * dt, 99);
+
+			if (progress < 99) {
+				progressRafId = requestAnimationFrame(tick);
+			}
+		}
+
+		progressRafId = requestAnimationFrame(tick);
+	}
+
+	function stopProgress() {
+		if (progressRafId) {
+			cancelAnimationFrame(progressRafId);
+			progressRafId = 0;
 		}
 	}
 
-	async function generateAudio() {
-		status = 'generating';
-		errorMsg = '';
-		try {
-		const res = await fetch(`/api/audio/${slug}/${chapter}`, {
-			method: 'POST',
-			headers: { 'Content-Type': 'application/json' },
-			body: JSON.stringify({ voice, speed })
+	async function finishProgress() {
+		stopProgress();
+		// Animate quickly to 100 %.
+		const step = () => {
+			progress = Math.min(progress + 8, 100);
+			if (progress < 100) {
+				progressRafId = requestAnimationFrame(step);
+			}
+		};
+		progressRafId = requestAnimationFrame(step);
+		// Wait for the short fill animation to look good.
+		await new Promise((r) => setTimeout(r, 200));
+		stopProgress();
+	}
+
+	// ── Core logic ────────────────────────────────────────────────────────────
+
+	async function tryPresign(): Promise<string | null> {
+		const params = new URLSearchParams({
+			slug,
+			n: String(chapter),
+			voice,
+			speed: String(speed)
 		});
-			if (!res.ok) throw new Error(`Generation failed: ${res.status}`);
-			// After generation, fetch the presigned URL
-			await loadAudio();
+		const res = await fetch(`/api/presign/audio?${params}`);
+		if (res.status === 404) return null;
+		if (!res.ok) throw new Error(`presign HTTP ${res.status}`);
+		const data = (await res.json()) as { url: string };
+		return data.url;
+	}
+
+	async function handlePlay() {
+		if (status === 'ready' && audioUrl) {
+			togglePlay();
+			return;
+		}
+
+		status = 'loading';
+		errorMsg = '';
+
+		try {
+			// Fast path: audio already exists.
+			const url = await tryPresign();
+			if (url) {
+				audioUrl = url;
+				status = 'ready';
+				// Auto-play once metadata loads.
+				requestAnimationFrame(() => audioEl?.play());
+				return;
+			}
+
+			// Slow path: trigger generation.
+			status = 'generating';
+			startProgress();
+
+			const res = await fetch(`/api/audio/${slug}/${chapter}`, {
+				method: 'POST',
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ voice, speed })
+			});
+			if (!res.ok) throw new Error(`Generation failed: HTTP ${res.status}`);
+
+			// Generation succeeded — fill the bar to 100 % then load.
+			await finishProgress();
+
+			const url2 = await tryPresign();
+			if (!url2) throw new Error('Audio generated but presign returned 404');
+			audioUrl = url2;
+			status = 'ready';
+			requestAnimationFrame(() => audioEl?.play());
 		} catch (e) {
+			stopProgress();
+			progress = 0;
 			status = 'error';
-			errorMsg = `Generation failed: ${e}`;
+			errorMsg = String(e);
 		}
 	}
 
@@ -99,36 +183,43 @@
 		<span class="text-sm text-zinc-300 font-medium">Audio Narration</span>
 	</div>
 
-	{#if status === 'idle'}
-		<div class="flex gap-2">
-			<button
-				onclick={loadAudio}
-				class="px-3 py-1.5 rounded bg-zinc-700 text-zinc-300 text-sm hover:bg-zinc-600 transition-colors"
-			>
-				Check for audio
-			</button>
-			<button
-				onclick={generateAudio}
-				class="px-3 py-1.5 rounded bg-amber-400 text-zinc-900 text-sm font-semibold hover:bg-amber-300 transition-colors"
-			>
-				Generate audio
-			</button>
-		</div>
+	{#if status === 'idle' || status === 'loading'}
+		<button
+			onclick={handlePlay}
+			disabled={status === 'loading'}
+			class="px-4 py-1.5 rounded bg-amber-400 text-zinc-900 text-sm font-semibold hover:bg-amber-300 transition-colors disabled:opacity-50 disabled:cursor-not-allowed flex items-center gap-2"
+		>
+			{#if status === 'loading'}
+				<svg class="w-3.5 h-3.5 animate-spin" fill="none" viewBox="0 0 24 24">
+					<circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
+					<path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z"></path>
+				</svg>
+			{:else}
+				<svg class="w-3.5 h-3.5 ml-0.5" fill="currentColor" viewBox="0 0 24 24">
+					<path d="M8 5v14l11-7z"/>
+				</svg>
+			{/if}
+			Play narration
+		</button>
 
-	{:else if status === 'loading' || status === 'generating'}
-		<div class="flex items-center gap-2 text-zinc-400 text-sm">
-			<svg class="w-4 h-4 animate-spin" fill="none" viewBox="0 0 24 24">
-				<circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
-				<path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4z"></path>
-			</svg>
-			{status === 'generating' ? 'Generating audio (this may take a few minutes)…' : 'Loading…'}
+	{:else if status === 'generating'}
+		<!-- Pseudo progress bar while Kokoro generates audio -->
+		<div class="space-y-2">
+			<p class="text-xs text-zinc-400">Generating narration…</p>
+			<div class="w-full h-1.5 bg-zinc-700 rounded-full overflow-hidden">
+				<div
+					class="h-full bg-amber-400 rounded-full transition-none"
+					style="width: {progress}%"
+				></div>
+			</div>
+			<p class="text-xs text-zinc-500 tabular-nums">{Math.round(progress)} %</p>
 		</div>
 
 	{:else if status === 'error'}
 		<div class="text-red-400 text-sm">
 			<p>{errorMsg || 'Failed to load audio.'}</p>
 			<button
-				onclick={() => { status = 'idle'; }}
+				onclick={() => { status = 'idle'; errorMsg = ''; }}
 				class="mt-1 text-xs underline text-zinc-400 hover:text-zinc-200"
 			>
 				Dismiss
@@ -150,7 +241,6 @@
 
 		<!-- Custom controls -->
 		<div class="flex flex-col gap-2">
-			<!-- Play/pause + time -->
 			<div class="flex items-center gap-3">
 				<button
 					onclick={togglePlay}