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- playback resume write-back: guard against out-of-order delivery (a superseded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- Tests: dirty-guard + flag-settle + per-key throttle scoping + superseded-push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/lib/channelsUrlState.ts

@@ -5,7 +5,7 @@
 // feed's URL-stripping can't delete them out from under the manager. Only non-default values are
 // emitted, so URLs stay clean. The manager's name search is deliberately EPHEMERAL (like the feed/plex
 // rail search) and stays OUT of the shared contract.
-import { parseSortState, type SortState } from "./columnSort";
+import { decodeSortParam, encodeSortParam, type SortState } from "./columnSort";
 import { type FilterMap } from "../components/tableHeader";
 
 // The content state S8 mirrors. `view` is the raw tab string (the provider clamps it); `sort` is the
@@ -20,18 +20,8 @@ export interface ChannelsUrlState {
 const KEYS = ["ctab", "csort", "cf"] as const;
 const VIEWS = ["subscribed", "discovery", "hidden", "blocked"];
 
-// Sort ⇄ `key:dir` (e.g. `subscribers:desc`). Column sort keys are simple words (no colon), so a plain
-// split is safe. There is no default sort (fallback null), so any sort emits.
-function encodeSort(s: SortState): string | null {
-  return s ? `${s.key}:${s.dir}` : null;
-}
-function decodeSort(raw: string): SortState {
-  const i = raw.indexOf(":");
-  const key = i === -1 ? raw : raw.slice(0, i);
-  const dir = i === -1 ? "asc" : raw.slice(i + 1);
-  return parseSortState({ key, dir });
-}
-
+// Sort ⇄ `key:dir` (e.g. `subscribers:desc`) via the shared codec in columnSort. There is no default
+// sort here (fallback null), so any sort emits — no default short-circuit needed.
 // The FilterMap mixes scalar column filters (priority=select, name=text, the sync chip) with array ones
 // (tags=multi), so it's carried as a compact JSON blob rather than a nested query string — JSON round-
 // trips both shapes exactly (and future multi columns for free), and the values are opaque enum/id
@@ -72,7 +62,7 @@ export function channelsParamUpdates(
   if (!state) return { ctab: null, csort: null, cf: null };
   return {
     ctab: state.view && state.view !== "subscribed" ? state.view : null,
-    csort: encodeSort(state.sort),
+    csort: encodeSortParam(state.sort),
     cf: encodeFilters(state.filters),
   };
 }
@@ -92,7 +82,7 @@ export function channelsParamsToState(
   const csort = params.get("csort");
   return {
     view: tab && VIEWS.includes(tab) ? tab : base.view,
-    sort: csort != null ? (decodeSort(csort) ?? base.sort) : base.sort,
+    sort: csort != null ? (decodeSortParam(csort) ?? base.sort) : base.sort,
     filters: params.has("cf") ? decodeFilters(params.get("cf") ?? "") : base.filters,
   };
 }