fix(plex): resolve deep-review findings on the Discover/Collections batch

Round of USER-triggered deep-review fixes (11 CONFIRMED + 1 PLAUSIBLE, all
verified) on the 22-commit branch:

- collections: split the mutate-then-resync error surface. A resync READ that
  fails after a successful Plex mutation no longer reports a false 422 "Plex
  write failed" + rollback; it returns 502 "sync pending" (the next full sync
  reconciles the mirror). New _write_or_pending_error helper; delete is exempt
  (no Plex read). Covered by new happy-path / 422-rollback / 502 tests.
- New-collection dup-name guard is now advisory only: existingNames is the
  search-scoped, all-library union, but the backend enforces uniqueness PER
  library — so it no longer blocks a valid same-name collection in another
  library, and the backend 409 is localized to nameTaken.
- member-remove now confirms before the write-through (matches delete-collection).
- add-title / new-collection library search render an error state on request
  failure instead of a misleading "No matches".
- history: seed navCounter past the current entry's persisted sfScroll on load
  so post-reload ids can't collide with back-stack entries and mis-restore scroll.
- PageScroller: LRU-cap the navId-keyed savedScroll map (was unbounded).
- share isIntrinsicReadOnly between the browse tile and the info page so the
  lock/edit affordance can't drift.

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

@@ -10,6 +10,7 @@
     "delete": "Delete",
     "enableEdit": "Enable editing",
     "removeItem": "Remove from collection",
+    "removeConfirm": "Remove \"{{title}}\" from this collection? This affects all users on Plex.",
     "readOnly": "Read-only",
     "empty": "This collection has no items.",
     "more": "More",

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

@@ -10,6 +10,7 @@
     "delete": "Törlés",
     "enableEdit": "Szerkesztés engedélyezése",
     "removeItem": "Eltávolítás a kollekcióból",
+    "removeConfirm": "Eltávolítod a(z) „{{title}}” elemet ebből a kollekcióból? Ez a Plexen minden felhasználót érint.",
     "readOnly": "Csak olvasható",
     "empty": "Ennek a kollekciónak nincsenek elemei.",
     "more": "Több",