refactor(server): extract createApp function for server initialization

- Replace Server.App() with Server.Default() for internal server access
- Extract server app creation into Server.createApp(opts) for testability
- Move CORS whitelist from module-level variable to function parameter
- Update all tests to use Server.Default() instead of Server.App()

packages/opencode/src/cli/cmd/tui/worker.ts

@@ -54,7 +54,7 @@ const startEventStream = (input: { directory: string; workspaceID?: string }) =>
     const request = new Request(input, init)
     const auth = getAuthorizationHeader()
     if (auth) request.headers.set("Authorization", auth)
-    return Server.App().fetch(request)
+    return Server.Default().fetch(request)
   }) as typeof globalThis.fetch
 
   const sdk = createOpencodeClient({
@@ -110,7 +110,7 @@ export const rpc = {
       headers,
       body: input.body,
     })
-    const response = await Server.App().fetch(request)
+    const response = await Server.Default().fetch(request)
     const body = await response.text()
     return {
       status: response.status,