feat(plex): add a movie or show to a collection from its card menu

"Add to collection…" opens a panel beside the card: the title's library's
manual collections tick it in or out, and a name + Enter creates a new one with
the title in it. It used to take the Collections page, a new collection, its
name and a title search, then another search per title. A lean endpoint lists
the collections with whether each holds the title. Admin only, as on the
title's page.

Also: a collection seeded with a show is made as a show collection (Plex type
2). Measured: as a movie one it came out empty and refused the show after.

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

@@ -384,5 +384,11 @@
     "byGenre": "Browse by genre",
     "becauseWatched": "Because you watched {{title}}",
     "empty": "Nothing to discover yet — run a Plex sync from the admin Config page."
+  },
+  "collectionAdd": {
+    "row": "Add to collection…",
+    "title": "Add to collection",
+    "none": "No collection to add to yet — name one below.",
+    "loadFailed": "Couldn't load the collections"
   }
 }

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

@@ -384,5 +384,11 @@
     "byGenre": "Böngészés műfaj szerint",
     "becauseWatched": "Mert megnézted: {{title}}",
     "empty": "Még nincs mit felfedezni — futtass egy Plex-szinkront az admin Konfiguráció oldalról."
+  },
+  "collectionAdd": {
+    "row": "Kollekcióhoz adás…",
+    "title": "Kollekcióhoz adás",
+    "none": "Még nincs kollekció, amihez adhatnád – adj meg egy nevet lent.",
+    "loadFailed": "Nem sikerült betölteni a kollekciókat"
   }
 }