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

src/SlpModularCms.Api/Program.cs

@@ -46,6 +46,12 @@ if (app.Environment.IsDevelopment())
 
 app.UseHttpsRedirection();
 
+// Serve the public website ('/') and the CMS admin SPA ('/admin') from wwwroot.
+// wwwroot/index.html + assets       -> public website (built and deployed separately, not part of this repo)
+// wwwroot/admin/index.html + assets -> CMS admin build (see frontend/, copied in on publish)
+app.UseDefaultFiles();
+app.UseStaticFiles();
+
 app.UseCors();
 
 // 7. Use Module Middleware
@@ -56,4 +62,9 @@ app.UseAuthorization();
 
 app.MapControllers();
 
+// SPA fallbacks so client-side routes (e.g. /admin/dashboard) resolve to the right index.html
+// instead of 404ing. The "nonfile" constraint keeps genuinely missing assets (e.g. /admin/assets/x.js) as 404s.
+app.MapFallbackToFile("/admin/{*path:nonfile}", "admin/index.html");
+app.MapFallbackToFile("{*path:nonfile}", "index.html");
+
 app.Run();