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- playback resume write-back: guard against out-of-order delivery (a superseded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- Tests: dirty-guard + flag-settle + per-key throttle scoping + superseded-push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

@@ -34,6 +34,22 @@ export function parseSortState(raw: unknown): SortState {
   return typeof key === "string" && (dir === "asc" || dir === "desc") ? { key, dir } : null;
 }
 
+/** Encode a SortState to the compact `key:dir` wire form the shareable-URL modules (plex/channels)
+ * put in the address bar. Sort keys are simple words (no colon), so the plain join round-trips. Returns
+ * null for no-sort. A module with a default sort short-circuits that to null at its own call site. */
+export function encodeSortParam(s: SortState): string | null {
+  return s ? `${s.key}:${s.dir}` : null;
+}
+
+/** Inverse of `encodeSortParam`: parse `key:dir` (or a bare key → asc) from the URL back into a
+ * SortState, healing anything malformed to null. One codec for both URL-state modules — no drift. */
+export function decodeSortParam(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 });
+}
+
 /** The header-click cycle: unsorted → ascending → descending → unsorted. */
 export function cycleSort(current: SortState, key: string): SortState {
   if (!current || current.key !== key) return { key, dir: "asc" };