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/AlbumBubble.tsx

@@ -5,6 +5,7 @@ import { createPortal } from "react-dom";
 import { useI18n } from "../../../i18n";
 import type { Attachment, Post } from "../../../types/post";
 import { AttachmentDownloadPill } from "../AttachmentDownloadPill";
+import { BubbleImage } from "../BubbleImage";
 import { useLightbox } from "../overlays/ImageLightbox";
 import { autolink } from "../utils/autolink";
 import { downloadAttachment } from "../utils/downloadFile";
@@ -126,9 +127,8 @@ function ImageListDialog({
                 className="flex min-w-0 flex-1 items-center gap-3 text-left"
               >
                 <div className="h-16 w-16 shrink-0 overflow-hidden rounded-lg bg-black">
-                  <img
+                  <BubbleImage
                     src={image.thumbnailUrl ?? image.url}
-                    alt=""
                     className="h-full w-full object-cover"
                   />
                 </div>
@@ -181,11 +181,10 @@ export function AlbumBubble({ post }: { post: Post }) {
                   else openLightbox(images, i, text, post.id);
                 }}
                 className="block h-full w-full"
-                aria-label={isLastSlot ? "Open image list" : att.filename}
+                aria-label={isLastSlot ? "Open image list" : "View image"}
               >
-                <img
+                <BubbleImage
                   src={att.thumbnailUrl ?? att.url}
-                  alt={att.filename}
                   loading="lazy"
                   className="h-full w-full object-cover"
                 />