feat: reskin admin pages to zine design system

Migrate the entire admin section from the dark guild-* Tailwind theme
to the zine design system (dashed borders, CSS custom properties,
Brygada 1918 + Commit Mono, cream/dark mode palette).

- Replace admin top-nav layout with sidebar matching default layout
- Reskin dashboard, members, events, series management pages
- Reskin events/create and series/create form pages
- Add dev-only test login endpoint (GET /api/dev/test-login)
- Redirect duplicate admin/dashboard.vue to /admin
- Update CLAUDE.md design system docs

nuxt.config.ts

@@ -3,6 +3,11 @@ export default defineNuxtConfig({
   compatibilityDate: "2025-07-15",
   devtools: { enabled: process.env.NODE_ENV !== "production" },
   modules: ["@nuxt/eslint", "@nuxt/ui", "@nuxtjs/plausible"],
+  colorMode: {
+    preference: "system",
+    fallback: "light",
+    classSuffix: "",
+  },
   app: {
     head: {
       link: [