User management: signing in is required before anything is loaded, the sign-in screen is drawn in the same canvas (user name, password as bullets, Enter moves on and submits) and the editor is built only after it; the API gained auth/session, auth/login and auth/logout while every other route answers 401 without a session, projects carry an owner and a foreign one comes back as 404, accounts live in data/users.json with a password hash and sessions as files in data/sessions with a sliding 30 days, so a build under the open tab does not sign anybody out; Logout sits at the far right of the toolbar and, as asked in the issue, does not reload the page: Editor.dispose() takes the editor down while the canvas stays, which is why the autosave, the focus handlers, the window listeners of the tilemap panel, the shortcuts and the plugins all got a way back; the tools sign in as e2e (tools/lib/auth.mjs, password from GAME_MAKER_E2E_PASSWORD or the git-ignored data/e2e-password.txt) and the browser gets the same session by cookie, new e2e module 45 covers the screen, the wrong pair, the bullets, logout without a reload and signing in again, new unit tests for HttpAuthStore and for what a session leaves behind (#177)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/store/AuthStore.ts

@@ -0,0 +1,19 @@
+/**
+ * Signing in and out. The token itself never reaches this code: the server keeps the session and
+ * hands the browser a cookie, so all that travels here is the name behind it.
+ */
+export interface AuthStore {
+  /** The signed-in user, or null while nobody is. */
+  session(): Promise<string | null>;
+  /** The user name on success; a rejected promise when the pair is wrong. */
+  logIn(username: string, password: string): Promise<string>;
+  logOut(): Promise<void>;
+}
+
+/** Thrown for a wrong user name or password, as opposed to a server that is not there. */
+export class WrongCredentialsError extends Error {
+  constructor(message = 'Wrong user name or password') {
+    super(message);
+    this.name = 'WrongCredentialsError';
+  }
+}