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/TopStrip.vue

@@ -5,15 +5,20 @@
     </span>
     <span>
       <slot name="right">
-        <template v-if="memberData">
-          Signed in as {{ memberData.name }}
-          <template v-if="memberData.circle">
-            &middot; {{ memberData.circle }}
+        <ClientOnly>
+          <template v-if="memberData">
+            Signed in as {{ memberData.name }}
+            <template v-if="memberData.circle">
+              &middot; {{ memberData.circle }}
+            </template>
           </template>
-        </template>
-        <template v-else>
-          A cooperative for game developers
-        </template>
+          <template v-else>
+            A cooperative for game developers
+          </template>
+          <template #fallback>
+            A cooperative for game developers
+          </template>
+        </ClientOnly>
       </slot>
     </span>
   </div>