fix(plex): round-2 deep-review follow-ups on the review fixes

- collections: the resync-failed-after-successful-write path now returns 422
  (honest "reconcile on next sync" detail), NOT 502. The frontend treats
  502/503/504 as gateway-unreachable and raises a sticky offline banner
  (api/core.ts RETRIABLE_GATEWAY); the app answered, so a 422 action-error
  toast is correct. Removes the false-offline regression the 502 introduced,
  and — since 422 isn't retried — the create double-write vector with it.
- tests: make the mutate-then-resync rollback assertion non-vacuous — the
  resync stub now flushes a partial local change before raising, so the test
  proves db.rollback() actually undoes it; split into a write-failed (422)
  case and a resync-failed (422 + reconcile detail + rollback) case.
- New-collection dup hint is now muted-tone with its own nameMaybeTaken copy
  (per-library uniqueness means the name may be free here) instead of the red
  authoritative nameTaken, which the still-enabled create flow contradicts.
- history: cover the navCounter post-reload collision guard (ensureNavId
  advances the counter past a higher persisted sfScroll id).

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

@@ -11,6 +11,7 @@
     "enableEdit": "Enable editing",
     "removeItem": "Remove from collection",
     "removeConfirm": "Remove \"{{title}}\" from this collection? This affects all users on Plex.",
+    "nameMaybeTaken": "A collection with this name may already exist in another library — you can still create it here.",
     "readOnly": "Read-only",
     "empty": "This collection has no items.",
     "more": "More",

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

@@ -11,6 +11,7 @@
     "enableEdit": "Szerkesztés engedélyezése",
     "removeItem": "Eltávolítás a kollekcióból",
     "removeConfirm": "Eltávolítod a(z) „{{title}}” elemet ebből a kollekcióból? Ez a Plexen minden felhasználót érint.",
+    "nameMaybeTaken": "Lehet, hogy már létezik ilyen nevű kollekció egy másik könyvtárban — itt akkor is létrehozhatod.",
     "readOnly": "Csak olvasható",
     "empty": "Ennek a kollekciónak nincsenek elemei.",
     "more": "Több",