Serve SPA from API and add health endpoint

Switch backend to serve the built SPA and static assets, add a minimal health endpoint, and clean up template code. Changes: added API/Controllers/HealthController.cs (GET /api/health -> 200), removed WeatherForecast controller and model, removed OpenAPI package reference and OpenAPI wiring from API.csproj/Program.cs, updated Program.cs to serve wwwroot with default files/static files and a SPA fallback that returns index.html for non-/api routes (/api/* returns 404), updated GUI/vite.config.ts to output build to API/wwwroot and clear the directory, added API/wwwroot to .gitignore, and updated codexInfo.md to document these changes. This enables the GUI build to be deployed directly into the API project and served as a single-page app.

API/Program.cs

@@ -1,23 +1,31 @@
 var builder = WebApplication.CreateBuilder(args);
 
-// Add services to the container.
-
 builder.Services.AddControllers();
-// Learn more about configuring OpenAPI at https://aka.ms/aspnet/openapi
-builder.Services.AddOpenApi();
 
 var app = builder.Build();
+var webRootPath = app.Environment.WebRootPath ?? Path.Combine(app.Environment.ContentRootPath, "wwwroot");
+var indexFilePath = Path.Combine(webRootPath, "index.html");
 
-// Configure the HTTP request pipeline.
-if (app.Environment.IsDevelopment())
-{
-    app.MapOpenApi();
-}
-
-app.UseHttpsRedirection();
-
-app.UseAuthorization();
+app.UseDefaultFiles();
+app.UseStaticFiles();
 
 app.MapControllers();
+app.MapFallback(async context =>
+{
+    if (context.Request.Path.StartsWithSegments("/api"))
+    {
+        context.Response.StatusCode = StatusCodes.Status404NotFound;
+        return;
+    }
+
+    if (!File.Exists(indexFilePath))
+    {
+        context.Response.StatusCode = StatusCodes.Status404NotFound;
+        return;
+    }
+
+    context.Response.ContentType = "text/html; charset=utf-8";
+    await context.Response.SendFileAsync(indexFilePath);
+});
 
 app.Run();