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

.gitignore

@@ -28,8 +28,8 @@ bld/
 
 # Visual Studio 2015/2017/2019/2022 cache/options directory
 .vs/
-# Uncomment if you have tasks that create the project's static files in wwwroot
-#wwwroot/
+# Generated on publish: admin SPA build (+ separately-deployed public website), see SlpModularCms.Api.csproj
+wwwroot/
 
 # Visual Studio Code design time settings
 .vscode/