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- playback resume write-back: guard against out-of-order delivery (a superseded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- Tests: dirty-guard + flag-settle + per-key throttle scoping + superseded-push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

@@ -131,15 +131,31 @@ function readPlexFilters(): PlexFilters {
     return EMPTY_PLEX_FILTERS;
   }
 }
-function readPlexUrlSeed(): PlexUrlState | null {
+// A shared-link seed = the URL-merged state PLUS which dimensions the URL actually carried. `present`
+// matters because the base below is read at MOUNT, before the account is pinned (getAccountRaw returns
+// null then), so the base for an OMITTED dimension is a hard default, NOT this account's stored value.
+// The provider must therefore persist/keep only the PRESENT dimensions on account-pin and re-read the
+// account's real stored value for the absent ones — otherwise a partial link silently resets them.
+type PlexSeedPresent = { scope: boolean; sort: boolean; show: boolean; filters: boolean };
+type PlexUrlSeed = { state: PlexUrlState; present: PlexSeedPresent };
+function readPlexUrlSeed(): PlexUrlSeed | null {
   const params = new URLSearchParams(window.location.search);
   if (!hasPlexParams(params)) return null;
-  return plexParamsToState(params, {
+  const state = plexParamsToState(params, {
     scope: readPlexScope(),
     sort: readPlexSort(),
     show: readPlexShow(),
     filters: readPlexFilters(),
   });
+  return {
+    state,
+    present: {
+      scope: params.has("pscope"),
+      sort: params.has("psort"),
+      show: params.has("pshow"),
+      filters: params.has("pf"),
+    },
+  };
 }
 
 export function PlexProvider({ children }: { children: ReactNode }) {
@@ -148,12 +164,12 @@ export function PlexProvider({ children }: { children: ReactNode }) {
   const [urlSeed] = useState(readPlexUrlSeed);
   // The former per-library picker is now a cross-library SCOPE: movie | show | both (unified library).
   // (Reuses the old LS.plexLibrary key; an old stored library-id value falls back to "both".)
-  const [scope, setScopeState] = useState<string>(() => urlSeed?.scope ?? readPlexScope());
-  const [show, setShowState] = useState<string>(() => urlSeed?.show ?? readPlexShow());
-  const [sort, setSortState] = useState<SortState>(() => urlSeed?.sort ?? readPlexSort());
+  const [scope, setScopeState] = useState<string>(() => urlSeed?.state.scope ?? readPlexScope());
+  const [show, setShowState] = useState<string>(() => urlSeed?.state.show ?? readPlexShow());
+  const [sort, setSortState] = useState<SortState>(() => urlSeed?.state.sort ?? readPlexSort());
   // The expanded Plex filters live as one JSON blob (the string-only account store serializes it).
   const [filtersRaw, setFiltersRaw] = useState<string>(() =>
-    urlSeed ? JSON.stringify(urlSeed.filters) : readPlexFiltersRaw(),
+    urlSeed ? JSON.stringify(urlSeed.state.filters) : readPlexFiltersRaw(),
   );
   const filters = useMemo<PlexFilters>(() => {
     try {
@@ -180,10 +196,20 @@ export function PlexProvider({ children }: { children: ReactNode }) {
   // non-shared slices (q/field) from storage. Mirrors FeedFiltersProvider's initialHadUrlFilters branch.
   const markDirty = useAccountRehydrate(() => {
     if (urlSeed) {
-      setAccountRaw(LS.plexLibrary, urlSeed.scope);
-      setAccountRaw(LS.plexShow, urlSeed.show);
-      setAccountRaw(LS.plexSort, JSON.stringify(urlSeed.sort));
-      setAccountRaw(LS.plexFilters, JSON.stringify(urlSeed.filters));
+      // Per dimension: a dimension the URL carried (present) is the shared view — adopt it and persist
+      // it to this now-pinned account (so it wins and survives a later plain reload). A dimension the
+      // URL OMITTED was seeded from a mount-time default (no account yet), so do NOT persist that
+      // default over the account's real stored value — re-read the stored value instead, exactly like a
+      // normal load. This is the fix for a partial shared link silently resetting the omitted slices.
+      const { state, present } = urlSeed;
+      if (present.scope) setAccountRaw(LS.plexLibrary, state.scope);
+      else setScopeState(readPlexScope());
+      if (present.show) setAccountRaw(LS.plexShow, state.show);
+      else setShowState(readPlexShow());
+      if (present.sort) setAccountRaw(LS.plexSort, JSON.stringify(state.sort));
+      else setSortState(readPlexSort());
+      if (present.filters) setAccountRaw(LS.plexFilters, JSON.stringify(state.filters));
+      else setFiltersRaw(readPlexFiltersRaw());
       setQState(readPlexQ());
       setFieldState(readPlexField());
       return;