style(frontend): Prettier config + one-time repo-wide format pass

Prettier 3 (pinned), printWidth 100, double quotes — chosen to match the existing
hand-formatting and minimise reflow. This commit is ONLY the mechanical format pass
(`prettier --write`) plus the config/ignore/scripts, isolated so it never pollutes a
feature diff. Verified behaviour-neutral: typecheck, eslint (0 errors), and vitest
all green before and after. See .git-blame-ignore-revs — `git blame` skips this sha.

frontend/src/components/Sidebar.tsx

@@ -95,14 +95,12 @@ export default function Sidebar({
   const facetsReady = !!facetsQuery.data;
   const facetCounts = facetsQuery.data?.counts ?? {};
   const chipCount = (tag: Tag): number =>
-    facetsReady ? facetCounts[String(tag.id)] ?? 0 : tag.channel_count;
+    facetsReady ? (facetCounts[String(tag.id)] ?? 0) : tag.channel_count;
   const visibleChips = (list: Tag[]): Tag[] => {
     const shown = facetsReady
       ? list.filter((tg) => chipCount(tg) > 0 || filters.tags.includes(tg.id))
       : list;
-    return [...shown].sort(
-      (a, b) => chipCount(b) - chipCount(a) || a.name.localeCompare(b.name)
-    );
+    return [...shown].sort((a, b) => chipCount(b) - chipCount(a) || a.name.localeCompare(b.name));
   };
 
   const languages = tags.filter((t) => t.category === "language");