fix(plex): round-2 deep-review follow-ups on the review fixes

- collections: the resync-failed-after-successful-write path now returns 422
  (honest "reconcile on next sync" detail), NOT 502. The frontend treats
  502/503/504 as gateway-unreachable and raises a sticky offline banner
  (api/core.ts RETRIABLE_GATEWAY); the app answered, so a 422 action-error
  toast is correct. Removes the false-offline regression the 502 introduced,
  and — since 422 isn't retried — the create double-write vector with it.
- tests: make the mutate-then-resync rollback assertion non-vacuous — the
  resync stub now flushes a partial local change before raising, so the test
  proves db.rollback() actually undoes it; split into a write-failed (422)
  case and a resync-failed (422 + reconcile detail + rollback) case.
- New-collection dup hint is now muted-tone with its own nameMaybeTaken copy
  (per-library uniqueness means the name may be free here) instead of the red
  authoritative nameTaken, which the still-enabled create flow contradicts.
- history: cover the navCounter post-reload collision guard (ensureNavId
  advances the counter past a higher persisted sfScroll id).

backend/app/routes/plex/collections.py

@@ -209,20 +209,22 @@ def _reject_duplicate_name(
 
 
 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)."""
+    """Map a PlexError raised inside a mutate-then-resync block to the honest message. If the mutating
+    call itself failed (``wrote`` False) the write never landed → "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; say the local copy will reconcile on the next sync.
+
+    Both use 422, NOT a 5xx: the frontend treats 502/503/504 as "gateway unreachable" and raises a
+    sticky offline banner (see api/core.ts RETRIABLE_GATEWAY). A 422 surfaces the detail as an ordinary
+    action-error toast, which is what both of these are — the app answered, it didn't go unreachable."""
     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})"
-            ),
+        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}")
+    else:
+        detail = f"Plex write failed: {e}"
+    return HTTPException(status_code=422, detail=detail)
 
 
 class CreateCollectionIn(BaseModel):

backend/tests/test_plex_collections_filter.py

@@ -238,35 +238,45 @@ def test_remove_item_writes_through_to_plex(db, monkeypatch):
         app.dependency_overrides.clear()
 
 
-def test_mutation_failure_returns_422_and_keeps_local_row(db, monkeypatch):
+def test_mutation_failure_returns_422_write_failed(db, monkeypatch):
+    # The Plex mutation itself raised → the write never landed → 422 "Plex write failed", and resync
+    # is never reached (stub.calls holds only the mutating call).
     _lib, user = _seed_editable(db)
     stub = _CollStub(fail_on="rename_collection")
     client = _admin_client(db, user, monkeypatch, stub)
     try:
         r = client.patch("/api/plex/collections/A", json={"title": "Renamed"})
         assert r.status_code == 422, r.text
+        assert "write failed" in r.json()["detail"].lower()
     finally:
         app.dependency_overrides.clear()
-    row = db.execute(select(PlexCollection).where(PlexCollection.rating_key == "A")).scalars().first()
-    assert row.title == "My Coll"  # rolled back — the write never landed
 
 
-def test_resync_failure_after_successful_write_returns_502(db, monkeypatch):
-    # The mutation landed on Plex; only the follow-up refresh READ failed → 502 "sync pending", NOT a
-    # false 422 "write failed". Regression guard for the mutate-then-resync error split.
+def test_resync_failure_after_successful_write_is_422_and_rolls_back(db, monkeypatch):
+    # The mutation landed on Plex; only the follow-up refresh READ failed. We must (a) NOT claim a
+    # write failure and NOT use a 5xx (which the frontend treats as offline) → 422 with a "reconcile
+    # on the next sync" detail, and (b) roll back any partial local upsert the resync had flushed.
     _lib, user = _seed_editable(db)
     stub = _CollStub()
 
     def _resync_boom(db_, plex, lib_, rk):
+        # Simulate resync having flushed a partial local change before its Plex read blew up — the
+        # route's rollback must undo it (this is what makes the rollback assertion non-vacuous).
+        row = db_.execute(select(PlexCollection).where(PlexCollection.rating_key == rk)).scalars().first()
+        row.title = "PARTIAL_SHOULD_ROLL_BACK"
+        db_.flush()
         raise PlexError("read failed")
 
     client = _admin_client(db, user, monkeypatch, stub, resync=_resync_boom)
     try:
         r = client.patch("/api/plex/collections/A", json={"title": "Renamed"})
-        assert r.status_code == 502, r.text
+        assert r.status_code == 422, r.text
+        assert "reconcile" in r.json()["detail"].lower()
         assert "rename_collection" in stub.calls  # the write DID happen
     finally:
         app.dependency_overrides.clear()
+    row = db.execute(select(PlexCollection).where(PlexCollection.rating_key == "A")).scalars().first()
+    assert row.title == "My Coll"  # the partial resync upsert was rolled back
 
 
 def test_show_collection_gets_montage_art_from_shows(db, monkeypatch):

frontend/src/components/plexBrowseLists.tsx

@@ -140,7 +140,7 @@ function NewCollectionPanel({
         className="glass-card w-full rounded-lg px-3 py-1.5 text-sm focus:outline-hidden focus:ring-2 focus:ring-accent/40"
       />
       {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.nameMaybeTaken")}</p>
       ) : (
         <p className="mt-1 mb-2 text-xs text-muted">{t("plex.collection.newHint")}</p>
       )}

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

@@ -11,6 +11,7 @@
     "enableEdit": "Enable editing",
     "removeItem": "Remove from collection",
     "removeConfirm": "Remove \"{{title}}\" from this collection? This affects all users on Plex.",
+    "nameMaybeTaken": "A collection with this name may already exist in another library — you can still create it here.",
     "readOnly": "Read-only",
     "empty": "This collection has no items.",
     "more": "More",

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

@@ -11,6 +11,7 @@
     "enableEdit": "Szerkesztés engedélyezése",
     "removeItem": "Eltávolítás a kollekcióból",
     "removeConfirm": "Eltávolítod a(z) „{{title}}” elemet ebből a kollekcióból? Ez a Plexen minden felhasználót érint.",
+    "nameMaybeTaken": "Lehet, hogy már létezik ilyen nevű kollekció egy másik könyvtárban — itt akkor is létrehozhatod.",
     "readOnly": "Csak olvasható",
     "empty": "Ennek a kollekciónak nincsenek elemei.",
     "more": "Több",

frontend/src/lib/history.nav.dom.test.ts

@@ -42,6 +42,16 @@ describe("nav scroll ids", () => {
     expect(second).toBeGreaterThan(first);
   });
 
+  it("ensureNavId advances the counter past a higher persisted id (post-reload collision guard)", () => {
+    // After a full reload the module counter resets to 0 while sfScroll ids persist in history.state.
+    // Landing on an entry whose id is above the counter must push the counter past it, so the next
+    // minted id can't duplicate a live back-stack entry and mis-restore its scroll.
+    const high = 999999;
+    window.history.replaceState({ sfScroll: high }, "");
+    expect(ensureNavId()).toBe(high); // returns the persisted id, doesn't reassign
+    expect(nextNavId()).toBeGreaterThan(high); // and the next mint clears it
+  });
+
   it("notifyNav fires subscribers until they unsubscribe", () => {
     const cb = vi.fn();
     const unsub = subscribeNav(cb);