Refactor project structure and update documentation. Replace PixiJS with plain DOM for UI rendering, enhance README with game features, and revise protocol documentation for HTTP API. Remove unused files and streamline client code for better maintainability.

src/HSchool.Server/Game/SchoolState.cs

@@ -0,0 +1,10 @@
+namespace HSchool.Server.Game;
+
+/// <summary>
+/// Immutable copy of a school, safe to hand to request threads. The live <c>School</c> object
+/// never leaves the loop thread.
+/// </summary>
+internal sealed record SchoolState(int Id, string Name, DateTime GameTime, bool Running, byte SpeedIndex);
+
+/// <summary>Everything the main menu needs in one read.</summary>
+internal sealed record SchoolsState(int MaxSchools, IReadOnlyList<SchoolState> Schools);