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/Modal.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useRef, type ReactNode } from "react";
 import { createPortal } from "react-dom";
+import { useTranslation } from "react-i18next";
 import { X } from "lucide-react";
 import { useBackToClose } from "../lib/history";
 
@@ -27,6 +28,7 @@ export default function Modal({
   children: ReactNode;
   maxWidth?: string;
 }) {
+  const { t } = useTranslation();
   // Browser/mouse Back closes the topmost modal instead of navigating away.
   useBackToClose(onClose);
 
@@ -76,7 +78,8 @@ export default function Modal({
           <h2 className="text-lg font-semibold leading-snug">{title}</h2>
           <button
             onClick={onClose}
-            title="Close"
+            title={t("common.close")}
+            aria-label={t("common.close")}
             className="shrink-0 p-1.5 rounded-lg text-muted hover:text-fg hover:bg-surface transition"
           >
             <X className="w-4 h-4" />