feat: compose executable frontend runtime

src/application/ports/auth-session-port.js

@@ -7,6 +7,9 @@
  *
  * @typedef {{
  *   getState(): SessionState,
+ *   subscribe(listener: () => void): () => void,
+ *   beginSignIn(returnTo?: string): Promise<void>,
+ *   signOut(): Promise<void>,
  *   attach(request: Request): Promise<Request>,
  *   recover(): Promise<"restored" | "no-session">,
  *   onUnauthenticated(): void