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- playback resume write-back: guard against out-of-order delivery (a superseded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- Tests: dirty-guard + flag-settle + per-key throttle scoping + superseded-push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/ChannelsProvider.dom.test.tsx

@@ -17,6 +17,7 @@ vi.mock("./NavigationProvider", () => ({
 }));
 
 import { ChannelsProvider, useChannels, useChannelsActions } from "./ChannelsProvider";
+import { setActiveAccount } from "../lib/api/core";
 
 const wrapper = ({ children }: { children: ReactNode }) => (
   <ChannelsProvider>{children}</ChannelsProvider>
@@ -99,4 +100,33 @@ describe("ChannelsProvider — S8 shareable URL", () => {
     act(() => result.current.setView("subscribed"));
     expect(new URLSearchParams(window.location.search).has("ctab")).toBe(false);
   });
+
+  it("a PARTIAL shared link (fresh tab) does not clobber the account's stored sort/filters", () => {
+    // Regression: on a fresh tab the account pins AFTER mount, so the seed's base for the OMITTED
+    // dimensions was read as defaults. Persisting the whole seed used to overwrite the account's real
+    // stored sort + filter map. The fix persists ONLY the URL-present dimensions.
+    localStorage.setItem(
+      "siftlode.channelSort.1",
+      JSON.stringify({ key: "subscribers", dir: "desc" }),
+    );
+    localStorage.setItem("siftlode.channelsFilterMap.1", JSON.stringify({ sync: "needsFull" }));
+    sessionStorage.removeItem("siftlode.activeAccount"); // account not pinned yet at mount
+    window.history.replaceState({ sfPage: "channels" }, "", "/?ctab=hidden"); // only the tab in the URL
+
+    const { result } = render();
+    act(() => setActiveAccount(1)); // App pins the account after /api/me
+
+    // The present dim (tab) is adopted from the link...
+    expect(result.current.view).toBe("hidden");
+    // ...but the omitted sort + filters keep the account's real stored values, in state AND storage.
+    expect(result.current.sort).toEqual({ key: "subscribers", dir: "desc" });
+    expect(result.current.filters).toEqual({ sync: "needsFull" });
+    expect(JSON.parse(localStorage.getItem("siftlode.channelSort.1")!)).toEqual({
+      key: "subscribers",
+      dir: "desc",
+    });
+    expect(JSON.parse(localStorage.getItem("siftlode.channelsFilterMap.1")!)).toEqual({
+      sync: "needsFull",
+    });
+  });
 });