Replace server actions with API routes and add SWR data-fetching hooks

- Convert discover, stats, status, and system-health server actions to
  proper API route handlers under `app/api/`; delete `lib/actions/explore.ts`,
  `lib/actions/settings.ts`, and `lib/actions/setup.ts`
- Add `use-discover`, `use-stats`, and `use-system-health` SWR hooks
  that call the new routes; update `command-palette`, `title-card`,
  `update-toast`, and `stats-display` to consume them
- Lift auth centering wrapper from individual login/register pages into
  `(auth)/layout.tsx`; switch both pages from `auth.api.getSession` to
  the cached `getSession()` helper
- Relocate setup wizard from `app/(auth)/setup/` to `app/setup/` (outside
  auth group) with dedicated `copy-button` and `refresh-button` client
  components
- Move `not-found.tsx` and `error.tsx` to app root so they apply
  globally instead of only within the pages route group

hooks/use-stats.ts

@@ -0,0 +1,18 @@
+import useSWR from "swr";
+import type { HistoryBucket, TimePeriod } from "@/lib/services/discovery";
+import { fetcher } from "@/lib/swr/fetcher";
+
+interface StatsResponse {
+  count: number;
+  history: HistoryBucket[];
+}
+
+export function useStats(type: "movies" | "episodes", period: TimePeriod) {
+  const { data } = useSWR<StatsResponse>(
+    `/api/stats?type=${type}&period=${period}`,
+    fetcher,
+    { revalidateOnFocus: false },
+  );
+
+  return data;
+}