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/lib/api/plex.ts

@@ -187,6 +187,12 @@ export interface PlexCollection {
   art?: string[]; // up to 2 poster rating_keys for the browse-tile montage (browse union only)
   total_runtime_s?: number; // total runtime of all items, for the tile sub-line (browse union only)
 }
+/** A collection Plex regenerates outside a user's control — a smart collection or an external auto-list
+ *  (imdb/tmdb/tvdb/trakt/…) — can never be edited. The single source of truth for the lock badge shown
+ *  on both the browse tile and the info page, so the two surfaces can't drift. */
+export function isIntrinsicReadOnly(c: Pick<PlexCollection, "smart" | "source">): boolean {
+  return !!c.smart || (!!c.source && c.source !== "collection");
+}
 // The Collection info page: the card fields + hero aggregates + the ordered members.
 export interface PlexCollectionDetail extends PlexCollection {
   content_rating?: string | null;