fix(media-ui): plug AudioContext leak + guard <img> against undefined src

Two latent bugs caught in the audit pass:

1. **AudioContext leak in AudioWaveform.**
   The lazy WebAudio decoder constructed an `AudioContext` per
   mount-with-audioUrl and never called `.close()`. Browsers cap the
   total per page (Chrome ~6, Firefox ~6) so a long-lived session that
   remounted waveforms enough times eventually hit
   `InvalidStateError: cannot create AudioContext` and silently
   stopped decoding.
   Fix: wrap `decodeAudioData` in try/finally and `close()` the
   context in the finally. Errors from close() are swallowed (best-
   effort cleanup).

2. **Undefined img src in ImageGenStream.**
   When `status=streaming` arrived before the first
   `snapshot.partialUrl` (very common during the SSE handshake),
   the component rendered <img src={undefined}> which browsers treat
   as a broken-image icon. Same for status=complete + missing
   finalUrl.
   Fix: compute `visibleSrc` once; only mount <img> if a source
   exists, otherwise show 'Waiting for first frame\u2026' placeholder.
   Also removed the dead `revealedAt` state \u2014 the prior 0.95/1
   opacity dance was imperceptible and contributed nothing.

3. **Bonus: AudioWaveform `bars` clamp.**
   `bars={0}` (or negative / fractional) divided by zero on the
   canvas slot math and rendered an empty waveform. Now clamped to
   `Math.max(1, Math.floor(barsProp))`.

Tests: 10 \u2192 12
  AudioWaveform   \u00b7 bars=0 doesn't crash + canvas still renders
  ImageGenStream  \u00b7 streaming without partialUrl shows placeholder
                    instead of <img src={undefined}>

packages/media-ui/src/AudioWaveform.tsx

@@ -39,7 +39,7 @@ export interface AudioWaveformProps {
 export function AudioWaveform({
   peaks: peaksProp,
   audioUrl,
-  bars = 96,
+  bars: barsProp = 96,
   progress = 0,
   onSeek,
   width = 480,
@@ -50,6 +50,9 @@ export function AudioWaveform({
   className,
   style,
 }: AudioWaveformProps) {
+  // Clamp to a sane minimum — zero or negative bars would divide-by-zero
+  // on canvas and produce an empty render.
+  const bars = Math.max(1, Math.floor(barsProp));
   const canvasRef = useRef<HTMLCanvasElement>(null);
   const titleId = useId();
   const [decoded, setDecoded] = useState<number[] | null>(null);
@@ -74,7 +77,13 @@ export function AudioWaveform({
             .webkitAudioContext;
         if (!Ctx) return;
         const ac = new Ctx();
-        const audio = await ac.decodeAudioData(buf);
+        let audio;
+        try {
+          audio = await ac.decodeAudioData(buf);
+        } finally {
+          // Free the AudioContext slot — browsers cap the count per page.
+          void ac.close?.().catch(() => {});
+        }
         if (cancelled) return;
         const channel = audio.getChannelData(0);
         const block = Math.max(1, Math.floor(channel.length / bars));

packages/media-ui/src/ImageGenStream.tsx

@@ -1,9 +1,4 @@
-import {
-  useEffect,
-  useState,
-  type CSSProperties,
-  type ReactNode,
-} from 'react';
+import { type CSSProperties, type ReactNode } from 'react';
 
 export type ImageGenStatus = 'idle' | 'streaming' | 'complete' | 'error';
 
@@ -60,13 +55,16 @@ export function ImageGenStream({
   style,
 }: ImageGenStreamProps) {
   const height = Math.round(width / Math.max(0.1, aspect));
-  const [revealedAt, setRevealedAt] = useState<number | null>(null);
-  useEffect(() => {
-    if (status === 'complete') setRevealedAt(Date.now());
-    else setRevealedAt(null);
-  }, [status, finalUrl]);
-
   const progress = Math.max(0, Math.min(1, snapshot?.progress ?? 0));
+  // Pick the visible source: prefer the final URL when complete, otherwise
+  // the partial. If neither is supplied yet the host gets the placeholder
+  // shell — never an <img> with `src={undefined}` (browser broken-image).
+  const visibleSrc =
+    status === 'complete'
+      ? finalUrl ?? snapshot?.partialUrl
+      : status === 'streaming'
+        ? snapshot?.partialUrl
+        : undefined;
 
   return (
     <figure
@@ -96,10 +94,10 @@ export function ImageGenStream({
         {status === 'idle' && (
           <Placeholder text="Ready to generate." />
         )}
-        {(status === 'streaming' || status === 'complete') && (
-          // eslint-disable-next-line @next/next/no-img-element
+        {(status === 'streaming' || status === 'complete') &&
+          (visibleSrc ? (
             <img
-            src={status === 'complete' ? finalUrl ?? snapshot?.partialUrl : snapshot?.partialUrl}
+              src={visibleSrc}
               alt={ariaLabel ?? 'Generated image'}
               data-testid="bl-image-gen-stream-img"
               style={{
@@ -107,11 +105,12 @@ export function ImageGenStream({
                 height: '100%',
                 objectFit: 'cover',
                 filter: status === 'streaming' ? 'blur(4px)' : 'none',
-              opacity: revealedAt ? 1 : 0.95,
-              transition: 'filter 200ms ease, opacity 200ms ease',
+                transition: 'filter 200ms ease',
               }}
             />
-        )}
+          ) : (
+            <Placeholder text="Waiting for first frame…" />
+          ))}
         {status === 'streaming' && (
           <div
             data-testid="bl-image-gen-stream-progress"

packages/media-ui/src/__tests__/media-ui.test.tsx

@@ -51,6 +51,13 @@ describe('AudioWaveform', () => {
     render(<AudioWaveform bars={16} width={120} height={40} />);
     expect(screen.getByTestId('bl-audio-waveform')).toBeDefined();
   });
+
+  it('clamps bars=0 to a safe minimum (no division-by-zero on canvas)', () => {
+    // Should not throw, should render a canvas.
+    expect(() => render(<AudioWaveform peaks={[0.5]} bars={0} />)).not.toThrow();
+    const canvas = screen.getByTestId('bl-audio-waveform').querySelector('canvas');
+    expect(canvas).not.toBeNull();
+  });
 });
 
 describe('ImageGenStream', () => {
@@ -82,6 +89,15 @@ describe('ImageGenStream', () => {
     const e = screen.getByTestId('bl-image-gen-stream-error');
     expect(e.textContent).toMatch(/boom/);
   });
+
+  it('streaming with no partialUrl renders a placeholder instead of an empty <img>', () => {
+    render(<ImageGenStream status="streaming" snapshot={{ progress: 0.1 }} />);
+    expect(screen.queryByTestId('bl-image-gen-stream-img')).toBeNull();
+    // The waiting placeholder uses the same testid as the idle one (Placeholder helper).
+    expect(screen.getByTestId('bl-image-gen-stream-idle').textContent).toMatch(/waiting/i);
+    // Progress strip still renders.
+    expect(screen.getByTestId('bl-image-gen-stream-progress')).toBeDefined();
+  });
 });
 
 describe('VideoPlayer', () => {