refactor(plex): split PlexBrowse god-file into cards + views + shared UI (E4 S4)

Pure behaviour-preserving code-motion — PlexBrowse 1751 -> 538 lines. The ~11 inlined
components are self-contained (every view is invoked with shell callbacks as props; no
view reaches back into shell state), so they just move to sibling files.

New files:
- lib/plexWatch.ts: toggleWatchState (shared by the grid + person page)
- components/plexCards.tsx: PlexPosterCard, SeasonCard, CastStrip, RelatedStrip, EpisodeCard
- components/plexViews.tsx: PlexInfoView, PlexPersonView, PlexShowView, PlexSeasonView
- lib/plexDetailUi.tsx (extended): BackBtn, ActionBtn, WatchedToggle, DetailHero, epLabel
  (the file's own remit — "shared detail-page UI")

PlexBrowse keeps the Sub router + PLEX_SUB_URL + PAGE + PlexBrowse() shell. Repointed
the only external importer (PlexPersonView.dom.test) to plexCards/plexViews.

Self-E2E (localdev): Plex grid renders (poster cards + episodes); The Expanse show-drill
renders the hero + action buttons + season cards; ?v=show URL-mirroring intact; 0 console
errors. Gate green: typecheck, knip, eslint (0 err), prettier, 247 vitest.

frontend/src/lib/plexWatch.ts

@@ -0,0 +1,22 @@
+import type { TFunction } from "i18next";
+import { api, type PlexCard } from "./api";
+import { notifyActionError } from "./actionErrors";
+
+// Persist a watch-state toggle from any Plex surface: flip via the caller's `optimistic` cache
+// updater, call the API, surface a failure (the click used to look dead on 403/404/429), then
+// `resync` (invalidate) — which corrects the optimistic flip to truth either way. Shared by the
+// browse grid and the person page, each of which owns a different cache shape.
+export async function toggleWatchState(
+  card: PlexCard,
+  t: TFunction,
+  handlers: { optimistic: (next: string) => void; resync: () => void },
+) {
+  const next = card.status === "watched" ? "new" : "watched";
+  handlers.optimistic(next);
+  try {
+    await api.plexSetState(card.id, next);
+  } catch (e) {
+    notifyActionError(e, t("plex.watchedFailed"));
+  }
+  handlers.resync();
+}