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.ts

@@ -56,6 +56,7 @@ export {
   plexFilterCount,
   coercePlexFilters,
   plexFiltersSignature,
+  isIntrinsicReadOnly,
 } from "./api/plex";
 import { messagingApi } from "./api/messaging";
 export type * from "./api/messaging";

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;

frontend/src/lib/history.ts

@@ -21,6 +21,15 @@ if (typeof window !== "undefined" && "scrollRestoration" in window.history) {
   window.history.scrollRestoration = "manual";
 }
 let navCounter = 0;
+// sfScroll ids are persisted into window.history.state and survive a full page reload, but this
+// module-level counter resets to 0 on every load. Seed it past the current entry's id so freshly
+// minted ids can't collide with ids already carried by pre-reload back-stack entries (ids are
+// assigned in forward order, so the current entry's id is ≥ every ancestor's) — a collision would
+// otherwise cross-contaminate PageScroller's per-entry scroll offsets after a reload + navigation.
+if (typeof window !== "undefined") {
+  const seed = (window.history.state as { sfScroll?: number } | null)?.sfScroll;
+  if (typeof seed === "number") navCounter = seed;
+}
 /** A fresh entry id — stamp it into the state of every pushState that opens a new main-content view. */
 export function nextNavId(): number {
   return ++navCounter;
@@ -29,7 +38,12 @@ export function nextNavId(): number {
  *  initial load, or an entry pushed without one — so the id survives Back/Forward. */
 export function ensureNavId(): number {
   const st = (window.history.state ?? {}) as { sfScroll?: number };
-  if (typeof st.sfScroll === "number") return st.sfScroll;
+  if (typeof st.sfScroll === "number") {
+    // Keep the counter ahead of any id we observe (e.g. a Back landed on a higher-id entry after a
+    // reload seeded us low) so the next mint never duplicates a live entry's id.
+    if (st.sfScroll > navCounter) navCounter = st.sfScroll;
+    return st.sfScroll;
+  }
   const id = nextNavId();
   window.history.replaceState({ ...st, sfScroll: id }, "");
   return id;