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/LanguageSwitcher.tsx

@@ -64,10 +64,7 @@ export default function LanguageSwitcher({
   }, [variant, open]);
 
   const menu = (
-    <div
-      ref={panelRef}
-      className="glass-menu w-40 rounded-xl p-1.5 animate-[popIn_0.16s_ease]"
-    >
+    <div ref={panelRef} className="glass-menu w-40 rounded-xl p-1.5 animate-[popIn_0.16s_ease]">
       {LANGUAGES.map((l) => (
         <button
           key={l.code}
@@ -103,10 +100,13 @@ export default function LanguageSwitcher({
         </button>
         {open &&
           createPortal(
-            <div className="z-rail" style={{ position: "fixed", left: pos.left, bottom: pos.bottom }}>
+            <div
+              className="z-rail"
+              style={{ position: "fixed", left: pos.left, bottom: pos.bottom }}
+            >
               {menu}
             </div>,
-            document.body
+            document.body,
           )}
       </>
     );