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/NavigationProvider.tsx

@@ -133,7 +133,8 @@ export function NavigationProvider({ children }: { children: ReactNode }) {
     // Drop any prior _ytScrape marker — the new search's source isn't known yet; Feed re-stamps it
     // once the results resolve, so a reload only restores a confirmed scrape-sourced search.
     const { _ytScrape: _drop, ...st } = window.history.state || {};
-    if (st._yt) window.history.replaceState({ ...st, _yt: q }, ""); // refine current search
+    if (st._yt)
+      window.history.replaceState({ ...st, _yt: q }, ""); // refine current search
     else window.history.pushState({ ...st, sfPage: "feed", _yt: q }, ""); // new sub-view entry
   }, []);