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- playback resume write-back: guard against out-of-order delivery (a superseded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- Tests: dirty-guard + flag-settle + per-key throttle scoping + superseded-push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

@@ -760,13 +760,24 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
           mid-playback stall or in-flight seek (see the state); `loadError` surfaces a fatal failure so
           it can't hide behind a running video. Spinner on load/stall, error text otherwise. */}
       {(!ready || buffering || loadError) && (
-        <div className="absolute inset-0 grid place-items-center px-6 text-center text-sm pointer-events-none">
+        <div
+          role="status"
+          aria-live="polite"
+          className="absolute inset-0 grid place-items-center px-6 text-center text-sm pointer-events-none"
+        >
           {loadError ? (
             <span className="max-w-md text-danger">{loadError}</span>
           ) : (
             <div className="flex flex-col items-center gap-3 text-white/80">
               <Loader2 className="w-10 h-10 animate-spin" aria-hidden />
-              {!ready && <span>{t("plex.player.loading")}</span>}
+              {/* Announce the stall to assistive tech. Initial load shows visible text; a mid-playback
+                  rebuffer/seek (ready, no error) has no visible label, so give it an sr-only one via the
+                  aria-live region so a screen reader distinguishes "buffering" from a frozen frame. */}
+              {!ready ? (
+                <span>{t("plex.player.loading")}</span>
+              ) : buffering ? (
+                <span className="sr-only">{t("plex.player.buffering")}</span>
+              ) : null}
             </div>
           )}
         </div>