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/Feed.tsx

@@ -255,7 +255,7 @@ export default function Feed({
               meta: {
                 kind: "video-hidden",
                 videoId: id,
-                title: v?.title ?? "this video",
+                title: v?.title ?? i18n.t("feed.thisVideo"),
                 channelId: v?.channel_id ?? "",
                 channelName: v?.channel_title ?? "",
               },
@@ -270,7 +270,7 @@ export default function Feed({
               meta: {
                 kind: "video-watched",
                 videoId: id,
-                title: v?.title ?? "this video",
+                title: v?.title ?? i18n.t("feed.thisVideo"),
                 channelId: v?.channel_id ?? "",
                 channelName: v?.channel_title ?? "",
               },