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/bubbles/ImageBubble.tsx

@@ -1,5 +1,6 @@
 import type { Post } from "../../../types/post";
 import { AttachmentDownloadPill } from "../AttachmentDownloadPill";
+import { BubbleImage } from "../BubbleImage";
 import { useLightbox } from "../overlays/ImageLightbox";
 
 export function ImageBubble({ post }: { post: Post }) {
@@ -12,11 +13,10 @@ export function ImageBubble({ post }: { post: Post }) {
         type="button"
         onClick={() => openLightbox([att], 0, undefined, post.id)}
         className="block h-full w-full"
-        aria-label={att.filename}
+        aria-label="View image"
       >
-        <img
+        <BubbleImage
           src={att.url}
-          alt={att.filename}
           loading="lazy"
           className="h-full w-full object-cover"
         />