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/channelColumns.tsx

@@ -13,6 +13,8 @@ export function subsColumn<T extends { subscriber_count: number | null }>(t: TFu
     nowrap: true,
     sortable: true,
     sortValue: (c) => c.subscriber_count ?? -1,
-    render: (c) => <span className="text-muted tabular-nums">{formatCountOrDash(c.subscriber_count)}</span>,
+    render: (c) => (
+      <span className="text-muted tabular-nums">{formatCountOrDash(c.subscriber_count)}</span>
+    ),
   };
 }