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- playback resume write-back: guard against out-of-order delivery (a superseded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- Tests: dirty-guard + flag-settle + per-key throttle scoping + superseded-push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

@@ -6,7 +6,7 @@
 // keys are Plex-unique (like `?v=`/`?pl`), so no other provider ever reads or clears them. Only non-
 // default values are emitted, so URLs stay clean.
 import { parsePlexFilters, plexFiltersToParams, type PlexFilters } from "./api";
-import { parseSortState, type SortState } from "./columnSort";
+import { decodeSortParam, encodeSortParam, type SortState } from "./columnSort";
 import { PLEX_SORT_FALLBACK } from "./plexSort";
 
 // The content state S4 mirrors. `sort` is the RAW provider SortState (scope-healed only at the point of
@@ -22,17 +22,11 @@ const KEYS = ["pscope", "psort", "pshow", "pf"] as const;
 const SCOPES = ["movie", "show", "both"];
 const SHOWS = ["all", "unwatched", "in_progress", "watched"];
 
-// Sort ⇄ `key:dir` (e.g. `year:desc`). Plex sort keys are simple words (no colon), so a plain split is
-// safe. The default (`title:asc`) emits nothing.
+// Sort ⇄ `key:dir` (e.g. `year:desc`) via the shared codec in columnSort; the Plex default (`title:asc`)
+// emits nothing.
 function encodeSort(s: SortState): string | null {
   if (!s || (s.key === PLEX_SORT_FALLBACK?.key && s.dir === PLEX_SORT_FALLBACK?.dir)) return null;
-  return `${s.key}:${s.dir}`;
-}
-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 });
+  return encodeSortParam(s);
 }
 
 /** The address-bar delta for the Plex content state: each namespaced key mapped to its emitted value,
@@ -61,7 +55,7 @@ export function plexParamsToState(params: URLSearchParams, base: PlexUrlState):
   const scope = params.get("pscope");
   const show = params.get("pshow");
   const psort = params.get("psort");
-  const sort = psort != null ? (decodeSort(psort) ?? base.sort) : base.sort;
+  const sort = psort != null ? (decodeSortParam(psort) ?? base.sort) : base.sort;
   return {
     scope: scope && SCOPES.includes(scope) ? scope : base.scope,
     sort,