feat: image error placeholder + scroll-to-top on navigation

Image bubbles previously used the raw filename as alt text, so a failed
asset load exposed the file name in the broken-image box. Add a reusable
BubbleImage that renders an empty alt and falls back to a neutral
placeholder (ImageOff icon) on error; use it in the album, image, and
image-with-text bubbles, and drop the filename from their aria-labels.

Also add a global ScrollToTop that resets the window on route change so
desktop navigation matches mobile (e.g. clicking a category card no
longer lands at the bottom of the new page). Hash navigations are skipped
so #post-<id> deep-link scrolling still works.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/components/messageStream/BubbleImage.tsx

@@ -0,0 +1,46 @@
+import { ImageOff } from "lucide-react";
+import { useEffect, useState } from "react";
+
+type BubbleImageProps = {
+  src: string | undefined;
+  className?: string;
+  loading?: "lazy" | "eager";
+};
+
+/**
+ * Thumbnail <img> for message bubbles. Renders with an empty alt (decorative)
+ * and, if the asset fails to load, falls back to a neutral placeholder instead
+ * of the browser's broken-image box — which would otherwise expose the raw
+ * file name via alt text.
+ */
+export function BubbleImage({ src, className, loading }: BubbleImageProps) {
+  const [failed, setFailed] = useState(false);
+
+  // Reset when the source changes so a reused element re-attempts the new src.
+  useEffect(() => {
+    setFailed(false);
+  }, [src]);
+
+  if (!src || failed) {
+    return (
+      <div
+        className={`flex items-center justify-center bg-gradient-to-br from-neutral-800 to-neutral-900 ${
+          className ?? ""
+        }`}
+        aria-hidden
+      >
+        <ImageOff className="h-8 w-8 text-neutral-600" />
+      </div>
+    );
+  }
+
+  return (
+    <img
+      src={src}
+      alt=""
+      loading={loading}
+      className={className}
+      onError={() => setFailed(true)}
+    />
+  );
+}