Serves public website, admin SPA, and API from a single host

Shared hosting (e.g. mijnhostingpartner.nl) typically allows only one
site/app-pool, so SlpModularCms.Api now serves everything itself:
'/' for the customer's public website (deployed separately, not part
of this repo), '/admin' for the CMS admin SPA, and '/api/v1' for the
API as before.

- Program.cs: static files from wwwroot + SPA fallbacks per path so
  client-side routing works for both frontends.
- frontend/: builds with base '/admin/' in production (dev unchanged),
  router basepath follows suit.
- SlpModularCms.Api.csproj: publish now builds the admin frontend and
  copies its output into wwwroot/admin automatically.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wi5qHAuq8UbzN4NLUFeKkJ

frontend/src/router.tsx

@@ -231,6 +231,8 @@ export const routeTree = rootRoute.addChildren([
 export const router = createRouter({
     routeTree,
     defaultPreload: 'intent',
+    // Matches vite.config.ts's `base`: '/admin/' in production builds, '/' in dev.
+    basepath: import.meta.env.BASE_URL,
     context: { auth: undefined as unknown as AuthContextValue },
 });