chore(initial-implementation): remove RCL boilerplate, close phase 1 tracking

Per Phase 1 reviewer notes — strips Component1.razor, ExampleJsInterop, and
the wwwroot template assets generated by 'dotnet new razorclasslib'. Phase 5
will populate Marathon.UI from scratch with the real layout, components, and
wwwroot/index.html for BlazorWebView. Build still green (0/0).

src/Marathon.UI/wwwroot/exampleJsInterop.js

@@ -1,6 +0,0 @@
-// This is a JavaScript module that is loaded on demand. It can export any number of
-// functions, and may import other JavaScript modules if required.
-
-export function showPrompt(message) {
-  return prompt(message, 'Type anything here');
-}