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/ChannelsProvider.tsx

@@ -209,7 +209,15 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
     [search, statusFilter, view, sort, filtersResetToken, focusName, focusToken],
   );
   const actions = useMemo<ChannelsActions>(
-    () => ({ setSearch, setStatusFilter, setView, setLayout, setSort, focusChannel, goToFullHistory }),
+    () => ({
+      setSearch,
+      setStatusFilter,
+      setView,
+      setLayout,
+      setSort,
+      focusChannel,
+      goToFullHistory,
+    }),
     [setSearch, setStatusFilter, setView, setLayout, setSort, focusChannel, goToFullHistory],
   );