fix(plex,channels): deep-review fixes — durable watch-sync + no shared-link clobber

Deep-review of the quickfix batch surfaced two data/preference-loss bugs plus
smaller items; all fixed with tests.

- watch_sync _pull_apply: don't let the Plex-authoritative pull overwrite a DIRTY
  local row (a change whose immediate push failed) — that stranded the value
  forever since _repush_dirty only retries dirty rows. Leave it dirty for re-push.
- PlexProvider / ChannelsProvider: a partial shared link opened in a fresh tab
  persisted mount-time DEFAULTS over the account's stored scope/sort/filters
  (base read before the account was pinned). Persist/keep only URL-present dims;
  re-hydrate the omitted ones from storage.
- playback resume write-back: guard against out-of-order delivery (a superseded
  older push now drops) and evict the throttle map so it can't grow unbounded.
- PlexPlayer: announce mid-playback buffering to assistive tech (role=status +
  sr-only label); new plex.player.buffering (HU+EN).
- columnSort: hoist the shared key:dir sort codec used by both URL-state modules.
- Tests: dirty-guard + flag-settle + per-key throttle scoping + superseded-push
  (backend); hold-repeat/suppress-click/unmount + partial-link no-clobber +
  refetch-doesn't-clobber-undo (frontend). Backend pytest 475, FE vitest 519.

frontend/src/components/PriorityStepper.dom.test.tsx

@@ -12,7 +12,10 @@ vi.mock("react-i18next", async (importOriginal) => {
   return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
 });
 
-afterEach(cleanup);
+afterEach(() => {
+  cleanup();
+  vi.useRealTimers(); // any fake-timer test resets here so it can't leak into the next
+});
 
 const raise = () => screen.getByRole("button", { name: "channels.row.raisePriority" });
 const lower = () => screen.getByRole("button", { name: "channels.row.lowerPriority" });
@@ -78,4 +81,57 @@ describe("PriorityStepper", () => {
     fireEvent.click(raise());
     expect(onPriority).toHaveBeenCalledWith(1);
   });
+
+  it("holding a step button auto-repeats (accelerating) and suppresses the trailing click", () => {
+    vi.useFakeTimers();
+    const onPriority = vi.fn();
+    render(<PriorityStepper priority={0} onPriority={onPriority} onSet={vi.fn()} />);
+
+    fireEvent.pointerDown(raise());
+    vi.advanceTimersByTime(349);
+    expect(onPriority).not.toHaveBeenCalled(); // a tap below the 350ms threshold doesn't repeat
+    vi.advanceTimersByTime(1); // cross the threshold → first auto-repeat tick
+    expect(onPriority).toHaveBeenCalledTimes(1);
+
+    vi.advanceTimersByTime(1000); // several accelerating repeats while held
+    const heldCount = onPriority.mock.calls.length;
+    expect(heldCount).toBeGreaterThan(3);
+    expect(onPriority.mock.calls.every(([d]) => d === 1)).toBe(true);
+
+    fireEvent.pointerUp(raise());
+    vi.advanceTimersByTime(1000);
+    expect(onPriority.mock.calls.length).toBe(heldCount); // stops repeating on release
+
+    fireEvent.click(raise()); // the click that trails a hold is swallowed (no double-count)
+    expect(onPriority.mock.calls.length).toBe(heldCount);
+  });
+
+  it("a quick tap (released before the hold threshold) still nudges exactly once", () => {
+    vi.useFakeTimers();
+    const onPriority = vi.fn();
+    render(<PriorityStepper priority={0} onPriority={onPriority} onSet={vi.fn()} />);
+
+    fireEvent.pointerDown(lower());
+    vi.advanceTimersByTime(100); // below the threshold → no auto-repeat armed
+    fireEvent.pointerUp(lower());
+    fireEvent.click(lower()); // the tap's click is NOT suppressed
+    expect(onPriority.mock.calls).toEqual([[-1]]);
+  });
+
+  it("clears the auto-repeat timer on unmount (no fire after the row is gone)", () => {
+    vi.useFakeTimers();
+    const onPriority = vi.fn();
+    const { unmount } = render(
+      <PriorityStepper priority={0} onPriority={onPriority} onSet={vi.fn()} />,
+    );
+
+    fireEvent.pointerDown(raise());
+    vi.advanceTimersByTime(350); // repeat armed + first tick
+    const before = onPriority.mock.calls.length;
+    expect(before).toBeGreaterThan(0);
+
+    unmount();
+    vi.advanceTimersByTime(1000);
+    expect(onPriority.mock.calls.length).toBe(before); // no post-unmount timer fire
+  });
 });