i18n(a11y): translate the modal + Welcome close buttons and the feed fallback (C-3.22)

House policy is HU+EN for every user-facing string. Modal's shared close button was a
hardcoded title="Close" (in every modal) — now t("common.close") + an aria-label.
Welcome's lightbox close aria-label and Feed's "this video" notification fallback are
translated too (new feed.thisVideo, HU+EN). The other Close hardcodes the review
listed were already fixed since.

frontend/src/components/Welcome.tsx

@@ -212,6 +212,7 @@ function Preview({
 // Full-size image viewer: a dimmed, blurred backdrop with the screenshot fit to the viewport
 // (aspect ratio preserved via object-contain). Closes on backdrop click, the ✕, or Escape.
 function Lightbox({ src, label, onClose }: { src: string; label: string; onClose: () => void }) {
+  const { t } = useTranslation();
   useEffect(() => {
     const onKey = (e: KeyboardEvent) => {
       if (e.key === "Escape") onClose();
@@ -235,7 +236,7 @@ function Lightbox({ src, label, onClose }: { src: string; label: string; onClose
       <button
         type="button"
         onClick={onClose}
-        aria-label="Close"
+        aria-label={t("common.close")}
         className="absolute top-4 right-4 p-2 rounded-full glass-card glass-hover text-fg"
       >
         <X className="w-5 h-5" />