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/i18n/locales/en/feed.json

@@ -45,6 +45,7 @@
   "undo": "Undo",
   "markedWatchedNamed": "Marked watched “{{title}}”",
   "markedWatched": "Marked watched",
+  "thisVideo": "this video",
   "unwatch": "Unwatch",
   "source": {
     "label": "Source",

frontend/src/i18n/locales/hu/feed.json

@@ -45,6 +45,7 @@
   "undo": "Visszavonás",
   "markedWatchedNamed": "Megnézettnek jelölve: „{{title}}”",
   "markedWatched": "Megnézettnek jelölve",
+  "thisVideo": "ez a videó",
   "unwatch": "Megnézés visszavonása",
   "source": {
     "label": "Forrás",