Add admin HK color customization fields to theme editor

- Add 6 new admin color DB keys (admin_canvas, admin_surface, admin_text,
  admin_text_muted, admin_border, admin_sidebar_bg) that override the
  derived admin palette
- Extract adminPaletteCss() from themePaletteCss() for reuse
- Generate admin CSS variables in both :root and html.dark with overrides
- Persist admin color settings via saveTheme action
- Add Admin panel (HK) section to /admin/theme with color pickers

src/app/admin/theme/page.tsx

@@ -47,6 +47,15 @@ const HEADINGS: { key: string; label: string; def: string }[] = [
   { key: "size_heading_h3", label: "H3", def: "18" },
 ];
 
+const ADMIN_FIELDS: { key: string; label: string; def: string }[] = [
+  { key: "admin_canvas", label: "Admin canvas", def: "#0f172a" },
+  { key: "admin_surface", label: "Admin surface", def: "#1e293b" },
+  { key: "admin_text", label: "Admin text", def: "#f1f5f9" },
+  { key: "admin_text_muted", label: "Admin muted text", def: "#94a3b8" },
+  { key: "admin_border", label: "Admin border", def: "#334155" },
+  { key: "admin_sidebar_bg", label: "Admin sidebar", def: "#0f172a" },
+];
+
 const ALL_COLORS = [...COLOR_FIELDS, ...BUTTON_FIELDS, ...GRADIENT_FIELDS];
 
 function toHex(value: string | null, def: string): string {
@@ -92,7 +101,9 @@ export default async function AdminTheme({
 
   const current = Object.fromEntries(
     await Promise.all(
-      ALL_COLORS.map(async (f) => [f.key, (await siteSettings.get(f.key, f.def)) ?? f.def] as const),
+      [...ALL_COLORS, ...ADMIN_FIELDS].map(
+        async (f) => [f.key, (await siteSettings.get(f.key, f.def)) ?? f.def] as const,
+      ),
     ),
   ) as Record<string, string>;
   const defaultPreset = PRESETS["Atom (golden)"];
@@ -190,6 +201,14 @@ export default async function AdminTheme({
           <div className="admin-card">{colorGrid(GRADIENT_FIELDS, current)}</div>
         </section>
 
+        <h2 className="mt-8 text-lg font-extrabold">Admin panel (HK)</h2>
+        <section className="mt-4">
+          <p className="text-xs text-[var(--admin-text-muted)] mb-2">
+            Colors for the Housekeeping admin panel. These override the defaults derived from light/dark mode palettes.
+          </p>
+          <div className="admin-card">{colorGrid(ADMIN_FIELDS, current)}</div>
+        </section>
+
         <h2 className="mt-8 text-lg font-extrabold">Dark mode</h2>
         <section className="mt-4">
           <h3 className="admin-section-title">{t("colours")}</h3>