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/lib/theme-presets.ts

@@ -0,0 +1,108 @@
+// Theme constants shared by the admin Theme page and its server action. Kept in
+// a plain module because a "use server" file may only export async functions.
+
+// The colour settings ThemeVars injects into :root. Editing these recolours the
+// whole site live (ThemeVars reads them on every request).
+export const THEME_COLOR_KEYS = [
+  "color_primary",
+  "color_background",
+  "color_surface",
+  "color_dropdown",
+  "color_navbar",
+  "color_navbar_text",
+  "color_text",
+  "color_text_muted",
+  "color_accent",
+  "button_primary_color",
+  "button_text_color",
+  "border_color",
+] as const;
+
+// Curated palettes (faithful to the atom theme presets). Each is a full set of
+// the colour keys above so applying one fully reskins the site.
+export const PRESETS: Record<string, Record<string, string>> = {
+  "Atom (golden)": {
+    color_primary: "#f59e0b",
+    color_background: "#f8fafc",
+    color_surface: "#ffffff",
+    color_dropdown: "#ffffff",
+    color_navbar: "#ffffff",
+    color_navbar_text: "#1e293b",
+    color_text: "#0f172a",
+    color_text_muted: "#64748b",
+    color_accent: "#10b981",
+    button_primary_color: "#f59e0b",
+    button_text_color: "#1e293b",
+    border_color: "#eeb425",
+  },
+  Midnight: {
+    color_primary: "#eeb425",
+    color_background: "#0f1117",
+    color_surface: "#171a21",
+    color_dropdown: "#1f242d",
+    color_navbar: "#171a21",
+    color_navbar_text: "#e5e7eb",
+    color_text: "#e5e7eb",
+    color_text_muted: "#9ca3af",
+    color_accent: "#22c55e",
+    button_primary_color: "#eeb425",
+    button_text_color: "#1a1a2e",
+    border_color: "#eeb425",
+  },
+  Ocean: {
+    color_primary: "#0ea5e9",
+    color_background: "#f0f9ff",
+    color_surface: "#ffffff",
+    color_dropdown: "#ffffff",
+    color_navbar: "#ffffff",
+    color_navbar_text: "#0c4a6e",
+    color_text: "#082f49",
+    color_text_muted: "#64748b",
+    color_accent: "#14b8a6",
+    button_primary_color: "#0ea5e9",
+    button_text_color: "#ffffff",
+    border_color: "#38bdf8",
+  },
+  Forest: {
+    color_primary: "#16a34a",
+    color_background: "#f3faf4",
+    color_surface: "#ffffff",
+    color_dropdown: "#ffffff",
+    color_navbar: "#ffffff",
+    color_navbar_text: "#14532d",
+    color_text: "#052e16",
+    color_text_muted: "#5f6b63",
+    color_accent: "#84cc16",
+    button_primary_color: "#16a34a",
+    button_text_color: "#ffffff",
+    border_color: "#22c55e",
+  },
+  Sunset: {
+    color_primary: "#f97316",
+    color_background: "#fff7ed",
+    color_surface: "#ffffff",
+    color_dropdown: "#ffffff",
+    color_navbar: "#ffffff",
+    color_navbar_text: "#7c2d12",
+    color_text: "#431407",
+    color_text_muted: "#78716c",
+    color_accent: "#ec4899",
+    button_primary_color: "#f97316",
+    button_text_color: "#ffffff",
+    border_color: "#fb923c",
+  },
+  Candy: {
+    color_primary: "#ec4899",
+    color_background: "#fdf2f8",
+    color_surface: "#ffffff",
+    color_dropdown: "#ffffff",
+    color_navbar: "#ffffff",
+    color_navbar_text: "#831843",
+    color_text: "#500724",
+    color_text_muted: "#9d6b80",
+    color_accent: "#a855f7",
+    button_primary_color: "#ec4899",
+    button_text_color: "#ffffff",
+    border_color: "#f472b6",
+  },
+};