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/components/plexBrowseLists.tsx

@@ -4,9 +4,16 @@ import type { TFunction } from "i18next";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Check, ListMusic, Lock, Plus, Search, X } from "lucide-react";
 import { qk } from "../lib/queryKeys";
-import { api, type PlexCard, type PlexCollection } from "../lib/api";
+import {
+  api,
+  HttpError,
+  isIntrinsicReadOnly,
+  type PlexCard,
+  type PlexCollection,
+} from "../lib/api";
 import { formatRuntime } from "../lib/format";
 import { notifyActionError } from "../lib/actionErrors";
+import { notify } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
 import { LoadingState, StateMessage } from "./QueryState";
 import PosterTile from "./PosterTile";
@@ -21,12 +28,6 @@ function collectionMeta(c: PlexCollection, t: TFunction): string {
   return parts.join(" · ");
 }
 
-// A read-only collection (a smart or external auto-list — imdb/tmdb/…) can never be edited; mark it up
-// front on the tile, mirroring the badge on the info page.
-function isReadOnlyCollection(c: PlexCollection): boolean {
-  return !!c.smart || (!!c.source && c.source !== "collection");
-}
-
 // The Playlist / Collection browse surfaces reached from the header pill (Piece 2): a grid of tiles,
 // each a way INTO that area — a playlist tile opens the playlist view, a collection tile filters the
 // title grid by that collection. Both reuse the existing list endpoints (plexPlaylists / plexCollections
@@ -87,8 +88,12 @@ function NewCollectionPanel({
   const [busy, setBusy] = useState(false);
   const debounced = useDebounced(term.trim(), 300);
   const trimmed = name.trim();
-  const dup = trimmed.length > 0 && existingNames.has(trimmed.toLowerCase());
-  const nameOk = trimmed.length > 0 && !dup;
+  // Advisory only: `existingNames` is the search-scoped, all-library collection union, whereas the
+  // backend enforces uniqueness PER library (Plex only conflates same-named collections within one
+  // section). So a match here is a hint, not a hard block — it must not gate creation, or a valid
+  // same-name collection in another library would be un-creatable. The backend's 409 is authoritative.
+  const dupHint = trimmed.length > 0 && existingNames.has(trimmed.toLowerCase());
+  const nameOk = trimmed.length > 0;
   const q = useQuery({
     queryKey: qk.plexLibrary("collection-new", debounced),
     queryFn: () => api.plexLibrary({ scope: "both", q: debounced, limit: 12 }),
@@ -103,7 +108,13 @@ function NewCollectionPanel({
       const col = await api.plexCreateCollection("", trimmed, card.id); // backend derives the library
       onCreated(col.id);
     } catch (e) {
-      notifyActionError(e, t("plex.collection.editFailed"));
+      // The backend is authoritative on per-library name uniqueness — localize its 409 to nameTaken
+      // (notifyActionError would otherwise surface the server's raw English detail).
+      if (e instanceof HttpError && e.status === 409) {
+        notify({ level: "error", message: t("plex.collection.nameTaken") });
+      } else {
+        notifyActionError(e, t("plex.collection.editFailed"));
+      }
       setBusy(false);
     }
   }
@@ -128,7 +139,7 @@ function NewCollectionPanel({
         aria-label={t("plex.collection.newCollection")}
         className="glass-card w-full rounded-lg px-3 py-1.5 text-sm focus:outline-hidden focus:ring-2 focus:ring-accent/40"
       />
-      {dup ? (
+      {dupHint ? (
         <p className="mt-1 mb-2 text-xs text-danger">{t("plex.collection.nameTaken")}</p>
       ) : (
         <p className="mt-1 mb-2 text-xs text-muted">{t("plex.collection.newHint")}</p>
@@ -146,6 +157,8 @@ function NewCollectionPanel({
       </div>
       {!nameOk || debounced.length === 0 ? null : q.isLoading ? (
         <LoadingState label={t("plex.loading")} />
+      ) : q.isError ? (
+        <p className="text-muted text-sm">{t("common.loadError")}</p>
       ) : results.length === 0 ? (
         <p className="text-muted text-sm">{t("plex.collection.addNoResults")}</p>
       ) : (
@@ -244,7 +257,7 @@ export function PlexCollectionsBrowse({ onPick }: { onPick: (id: string, title:
               art={c.art}
               subtitle={collectionMeta(c, t)}
               topRight={
-                isReadOnlyCollection(c) ? (
+                isIntrinsicReadOnly(c) ? (
                   <Lock
                     className="h-3.5 w-3.5 text-white/80 drop-shadow"
                     aria-label={t("plex.collection.readOnly")}