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).

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);