Add Theme Settings tool + polish remaining radio detail pages

- New /admin/theme: recolour the whole site from housekeeping. 6 atom-
  faithful presets (Atom/Midnight/Ocean/Forest/Sunset/Candy) + per-colour
  pickers for the 12 settings ThemeVars injects + border radius. Writes to
  website_settings, busts the siteSettings cache, and revalidates the
  layout so the new palette applies live with no rebuild. Constants live
  in src/lib/theme-presets.ts (a "use server" file can't export objects).
  Added to the admin sidebar (System).
- radio/contests/[id] + giveaways/[id] wrapped in ContentCard to match
  the public design system.
- Skipped a separate VPN page: /admin/ip already manages the IP
  white/blacklist, so it would only duplicate it.

Verified live (prod, amx_test): applied the Ocean preset → home renders
--color-primary #0ea5e9 site-wide; reverted the test rows. tsc 0,
vitest 49/49, next build 0.

src/actions/admin-theme.ts

@@ -0,0 +1,65 @@
+"use server";
+
+import { revalidatePath } from "next/cache";
+import { redirect } from "next/navigation";
+import { requireStaff } from "@/lib/admin/guard";
+import { prisma } from "@/lib/prisma";
+import { siteSettings } from "@/lib/services/site-settings";
+import { logStaffActivity } from "@/lib/services/staff-activity";
+import { PRESETS, THEME_COLOR_KEYS } from "@/lib/theme-presets";
+
+// Only hex/keyword colour values are accepted (matches ThemeVars' sanitiser).
+const COLOR_RE = /^[#a-zA-Z0-9(),.\s%-]+$/;
+
+async function writeSetting(key: string, value: string): Promise<void> {
+  await prisma.websiteSetting.upsert({
+    where: { key },
+    update: { value },
+    create: { key, value, comment: "Theme (housekeeping)" },
+  });
+}
+
+export async function saveTheme(formData: FormData): Promise<void> {
+  const staff = await requireStaff();
+
+  try {
+    for (const key of THEME_COLOR_KEYS) {
+      const raw = String(formData.get(key) ?? "").trim();
+      if (raw && COLOR_RE.test(raw)) await writeSetting(key, raw);
+    }
+    const radius = String(formData.get("border_radius") ?? "").trim();
+    if (/^\d{1,3}$/.test(radius)) await writeSetting("border_radius", radius);
+
+    siteSettings.reload();
+    await logStaffActivity({
+      staffId: staff.id,
+      action: "theme_update",
+      description: "Updated theme colours",
+    });
+    revalidatePath("/", "layout");
+  } catch {
+    // ignore — page re-renders current state
+  }
+  redirect("/admin/theme?saved=1");
+}
+
+export async function applyPreset(formData: FormData): Promise<void> {
+  const staff = await requireStaff();
+  const name = String(formData.get("preset") ?? "");
+  const preset = PRESETS[name];
+  if (!preset) redirect("/admin/theme");
+
+  try {
+    for (const [key, value] of Object.entries(preset)) await writeSetting(key, value);
+    siteSettings.reload();
+    await logStaffActivity({
+      staffId: staff.id,
+      action: "theme_preset",
+      description: `Applied theme preset "${name}"`,
+    });
+    revalidatePath("/", "layout");
+  } catch {
+    // ignore
+  }
+  redirect(`/admin/theme?preset=${encodeURIComponent(name)}`);
+}