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.

backend/app/routes/plex/collections.py

@@ -208,6 +208,23 @@ def _reject_duplicate_name(
         raise HTTPException(status_code=409, detail="A collection with this name already exists")
 
 
+def _write_or_pending_error(wrote: bool, e: Exception) -> HTTPException:
+    """Map a PlexError raised inside a mutate-then-resync block to the honest status. If the mutating
+    call itself failed (``wrote`` False) the write never landed → 422 "Plex write failed". If the
+    write succeeded but the follow-up resync READ (Plex metadata/children) failed, the change IS on
+    the Plex server → don't claim a write failure; report 502 "sync pending" (the next full sync
+    reconciles the local mirror)."""
+    if wrote:
+        return HTTPException(
+            status_code=502,
+            detail=(
+                "Collection updated in Plex, but refreshing the local copy failed; "
+                f"it will reconcile on the next sync ({e})"
+            ),
+        )
+    return HTTPException(status_code=422, detail=f"Plex write failed: {e}")
+
+
 class CreateCollectionIn(BaseModel):
     library: str = ""
     title: str = ""
@@ -239,15 +256,17 @@ def create_collection(
     if lib is None:
         raise HTTPException(status_code=404, detail="Unknown library")
     _reject_duplicate_name(db, lib.id, title)
+    wrote = False
     try:
         with PlexClient(db) as plex:
             new = plex.create_collection(lib.plex_key, title, seed)
+            wrote = True
             col = plex_sync.resync_collection(db, plex, lib, str(new.get("ratingKey")))
             col.editable = True
         db.commit()
     except (PlexError, PlexNotConfigured) as e:
         db.rollback()
-        raise HTTPException(status_code=422, detail=f"Plex write failed: {e}")
+        raise _write_or_pending_error(wrote, e)
     return _collection_card(col)
 
 
@@ -260,14 +279,16 @@ def collection_add_item(
 ) -> dict:
     col = _editable_collection_or_403(db, rating_key)
     lib = db.get(PlexLibrary, col.library_id)
+    wrote = False
     try:
         with PlexClient(db) as plex:
             plex.add_collection_item(rating_key, str(item_rating_key))
+            wrote = True
             col = plex_sync.resync_collection(db, plex, lib, rating_key)
         db.commit()
     except (PlexError, PlexNotConfigured) as e:
         db.rollback()
-        raise HTTPException(status_code=422, detail=f"Plex write failed: {e}")
+        raise _write_or_pending_error(wrote, e)
     return _collection_card(col)
 
 
@@ -280,14 +301,16 @@ def collection_remove_item(
 ) -> dict:
     col = _editable_collection_or_403(db, rating_key)
     lib = db.get(PlexLibrary, col.library_id)
+    wrote = False
     try:
         with PlexClient(db) as plex:
             plex.remove_collection_item(rating_key, str(item_rating_key))
+            wrote = True
             col = plex_sync.resync_collection(db, plex, lib, rating_key)
         db.commit()
     except (PlexError, PlexNotConfigured) as e:
         db.rollback()
-        raise HTTPException(status_code=422, detail=f"Plex write failed: {e}")
+        raise _write_or_pending_error(wrote, e)
     return _collection_card(col)
 
 
@@ -308,14 +331,16 @@ def collection_rename(
         raise HTTPException(status_code=400, detail="title is required")
     _reject_duplicate_name(db, col.library_id, title, exclude_rk=rating_key)
     lib = db.get(PlexLibrary, col.library_id)
+    wrote = False
     try:
         with PlexClient(db) as plex:
             plex.rename_collection(lib.plex_key, rating_key, title)
+            wrote = True
             col = plex_sync.resync_collection(db, plex, lib, rating_key)
         db.commit()
     except (PlexError, PlexNotConfigured) as e:
         db.rollback()
-        raise HTTPException(status_code=422, detail=f"Plex write failed: {e}")
+        raise _write_or_pending_error(wrote, e)
     return _collection_card(col)