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/ScrollToTop.tsx

@@ -0,0 +1,22 @@
+import { useEffect } from "react";
+import { useLocation } from "react-router-dom";
+
+/**
+ * Resets the window to the top on every route change. React Router does not
+ * restore scroll on client navigation, so without this a short new page would
+ * clamp to wherever the previous (taller) page was scrolled — e.g. landing at
+ * the bottom of a category page after clicking a card far down the home grid.
+ *
+ * Skips navigations that carry a hash (`#post-<id>`, `#categories`, …) so
+ * anchor / deep-link targets keep their own scroll handling.
+ */
+export function ScrollToTop() {
+  const { pathname, hash } = useLocation();
+
+  useEffect(() => {
+    if (hash) return;
+    window.scrollTo({ top: 0, left: 0 });
+  }, [pathname, hash]);
+
+  return null;
+}