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

app/api/discover/route.ts

@@ -0,0 +1,83 @@
+import { type NextRequest, NextResponse } from "next/server";
+import { getSession } from "@/lib/auth/session";
+import { isTmdbConfigured } from "@/lib/config";
+import {
+  getEpisodeProgressByTmdbIds,
+  getUserStatusesByTmdbIds,
+} from "@/lib/services/tracking";
+import { discover } from "@/lib/tmdb/client";
+import { tmdbImageUrl } from "@/lib/tmdb/image";
+
+export async function GET(req: NextRequest) {
+  const session = await getSession();
+  if (!session) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  if (!isTmdbConfigured()) {
+    return NextResponse.json(
+      { error: "TMDB API key is not configured." },
+      { status: 503 },
+    );
+  }
+
+  const mediaType = req.nextUrl.searchParams.get("mediaType");
+  const genreId = req.nextUrl.searchParams.get("genreId");
+
+  if (mediaType !== "movie" && mediaType !== "tv") {
+    return NextResponse.json(
+      { error: "mediaType must be movie or tv" },
+      { status: 400 },
+    );
+  }
+
+  const parsedGenreId = Number(genreId);
+  if (!genreId || !Number.isFinite(parsedGenreId)) {
+    return NextResponse.json(
+      { error: "genreId must be a number" },
+      { status: 400 },
+    );
+  }
+
+  try {
+    const results = await discover(mediaType, {
+      sort_by: "popularity.desc",
+      "vote_count.gte": "50",
+      with_genres: String(parsedGenreId),
+    });
+
+    type DiscoverResult = NonNullable<typeof results.results>[number] & {
+      title?: string;
+      name?: string;
+      release_date?: string;
+      first_air_date?: string;
+    };
+
+    const items = ((results.results ?? []) as DiscoverResult[])
+      .filter((r) => r.poster_path)
+      .map((r) => ({
+        tmdbId: r.id,
+        type: mediaType,
+        title: r.title ?? r.name ?? "",
+        posterPath: tmdbImageUrl(r.poster_path ?? null, "posters"),
+        releaseDate: r.release_date ?? r.first_air_date ?? null,
+        voteAverage: r.vote_average,
+      }));
+
+    const lookups = items.map((r) => ({ tmdbId: r.tmdbId, type: r.type }));
+    const [userStatuses, episodeProgress] =
+      lookups.length > 0
+        ? await Promise.all([
+            getUserStatusesByTmdbIds(session.user.id, lookups),
+            getEpisodeProgressByTmdbIds(session.user.id, lookups),
+          ])
+        : [{}, {}];
+
+    return NextResponse.json({ items, userStatuses, episodeProgress });
+  } catch {
+    return NextResponse.json(
+      { error: "Failed to fetch discover results" },
+      { status: 502 },
+    );
+  }
+}