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/actions/admin-theme.ts

@@ -34,6 +34,12 @@ export async function saveTheme(formData: FormData): Promise<void> {
         if (raw && COLOR_RE.test(raw)) await writeSetting(dbKey, raw);
       }
     }
+    const ADMIN_KEYS = ["admin_canvas", "admin_surface", "admin_text", "admin_text_muted", "admin_border", "admin_sidebar_bg"];
+    for (const key of ADMIN_KEYS) {
+      const raw = String(formData.get(key) ?? "").normalize("NFC").trim();
+      if (raw && COLOR_RE.test(raw)) await writeSetting(key, raw);
+    }
+
     const radius = String(formData.get("border_radius") ?? "").normalize("NFC").trim();
     if (/^\d{1,3}$/.test(radius)) await writeSetting("border_radius", radius);