feat(i18n): translate login and app chrome (HU/EN/DE)

Login screen (with a language picker), header, account menu, sync status, About and
Release Notes dialogs, and the version banner are now fully translated in Hungarian,
English and German.

frontend/src/components/VersionBanner.tsx

@@ -1,10 +1,12 @@
 import { useState } from "react";
+import { useTranslation } from "react-i18next";
 import { Sparkles, X } from "lucide-react";
 import { FRONTEND_VERSION, SEEN_VERSION_KEY } from "../lib/version";
 
 // Eye-catching, dismissible bar shown once after the running build's version changes
 // (compares the baked frontend version to the last one the user acknowledged).
 export default function VersionBanner({ onOpen }: { onOpen: () => void }) {
+  const { t } = useTranslation();
   const [dismissed, setDismissed] = useState(false);
   const seen = localStorage.getItem(SEEN_VERSION_KEY);
   // Don't nag in un-stamped dev builds, or once this version has been seen.
@@ -19,9 +21,7 @@ export default function VersionBanner({ onOpen }: { onOpen: () => void }) {
   return (
     <div className="shrink-0 flex items-center gap-2 px-4 py-2 text-sm bg-accent/15 border-b border-accent/30 text-fg">
       <Sparkles className="w-4 h-4 text-accent shrink-0" />
-      <span className="min-w-0">
-        Updated to <span className="font-semibold">v{FRONTEND_VERSION}</span> — see what's changed.
-      </span>
+      <span className="min-w-0">{t("header.banner.updated", { version: FRONTEND_VERSION })}</span>
       <button
         onClick={() => {
           onOpen();
@@ -29,11 +29,11 @@ export default function VersionBanner({ onOpen }: { onOpen: () => void }) {
         }}
         className="ml-1 px-2.5 py-1 rounded-lg font-semibold bg-accent text-accent-fg hover:opacity-90 transition"
       >
-        Release notes
+        {t("header.banner.releaseNotes")}
       </button>
       <button
         onClick={markSeen}
-        title="Dismiss"
+        title={t("header.banner.dismiss")}
         className="ml-auto p-1 rounded-md text-muted hover:text-fg hover:bg-card transition"
       >
         <X className="w-4 h-4" />