feat(plex): add a "Recently watched" rail to Discover (D)

A new Discover rail listing what the user last FULLY watched, newest first: a movie as its
own card, a show as the specific EPISODE watched (carrying its show title via leaf_card). Sits
above Recently added. Movies honour the sidebar filters like the other rails; episodes carry no
filterable metadata so they don't. Clicking a card plays it (the existing onCard path already
routes episodes to the player). Bilingual heading added.

Verified live: the rail renders watched movies + episodes with Watched badges.

Gate: tsc/lint/knip/prettier clean, 423 vitest; backend ruff clean, 13 discover tests (+ a
recently-watched movie+episode assertion).

frontend/src/i18n/locales/en/plex.json

@@ -292,6 +292,7 @@
   "discover": {
     "tab": "Discover",
     "continueWatching": "Continue watching",
+    "recentlyWatched": "Recently watched",
     "recentlyAdded": "Recently added",
     "surprise": "Surprise me",
     "reshuffle": "Reshuffle",

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

@@ -292,6 +292,7 @@
   "discover": {
     "tab": "Felfedezés",
     "continueWatching": "Folytatás",
+    "recentlyWatched": "Nemrég megnézve",
     "recentlyAdded": "Nemrég hozzáadva",
     "surprise": "Lepj meg",
     "reshuffle": "Újrakeverés",