createApp function with context and onError

[qantrepreneur]

What to build

Implement the createApp function — the root configuration entry point for framework users. It accepts a config object with context (a function that receives a Request and returns custom context) and onError (a global error handler).

export default createApp({
  context: (request) => ({
    sdk: new SDK({ apiKey: "..." }),
  }),
  onError: (error, request) => {
    console.error(error);
    return new Response("Internal Server Error", { status: 500 });
  },
});

The return type of context should propagate through the type system so that ctx in loaders is fully typed with the user's custom properties + route params.

Acceptance criteria

• createApp accepts { context, onError } config
• context function receives Request and returns user-defined context
• onError is optional, receives (error: unknown, request: Request)
• Return type preserves the context type for downstream inference
• Unit tests for createApp with and without onError

Blocked by

• Core types: Context, SlotMap, TemplateRegistry, RouteMap #2