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

app/components/ParchmentInset.vue

@@ -23,7 +23,7 @@
 
 .parchment-inset :deep(p) {
   font-size: 13px;
-  color: #b8ae98;
+  color: var(--parch-text-dim);
   line-height: 1.75;
   max-width: 560px;
   margin-bottom: 12px;