Implement Workstream M: Admin Pages (Frontend)

Add superuser admin interface for managing organizations and users:
- Admin layout with access control (redirects non-superusers)
- Admin dashboard with org/user counts and quick actions
- Org management: list, create, view/edit details, manage sites
- User management: list, view details, toggle superuser, confirm email
- SuperuserBadge component for consistent superuser indication
- Sidebar shows admin link (shield icon) for superusers only
- Centralized date formatting utility at $lib/utils/format-date.ts
- Test plan documentation at docs/test-plans/admin.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/initial-app.md

@@ -2363,14 +2363,24 @@ _Depends on: J1-J6, C3_
 - Reusable components: `$lib/components/org/role-badge.svelte`, `confirm-dialog.svelte`
 - Sidebar updated with "Organizations" nav item
 
-#### Workstream M: Admin Pages (Frontend)
+#### Workstream M: Admin Pages (Frontend) ✅
 
 _Depends on: K1-K5, C3_
 _Can run parallel to L_
 
-- [ ] **M1**: Create `/admin` dashboard page
-- [ ] **M2**: Create `/admin/orgs` pages (list, new, details)
-- [ ] **M3**: Create `/admin/users` pages (list, details)
+- [x] **M1**: Create `/admin` dashboard page
+- [x] **M2**: Create `/admin/orgs` pages (list, new, details)
+- [x] **M3**: Create `/admin/users` pages (list, details)
+
+**Implementation notes:**
+- Admin layout at `/routes/admin/+layout.svelte` provides superuser access control
+- Redirects non-superusers to `/dashboard` with toast error
+- Admin dashboard shows org/user counts with quick action links
+- Org management: list all orgs, create new with owner email, view/edit details, manage sites
+- User management: list all users, view details, toggle superuser status, confirm email
+- Sidebar shows admin link (shield icon) only for superusers
+- Reusable component: `$lib/components/admin/superuser-badge.svelte`
+- All destructive actions use ConfirmDialog
 
 ---