Vendor backend and frontend into the monorepo

The submodule collapse was only half applied: .gitmodules was deleted but
backend/ and frontend/ were still recorded as gitlinks, so none of their
files were tracked. Replace the gitlinks with the real trees.

Also untrack .env (it carried placeholder secrets) in favour of a tracked
.env.example, drop the committed __pycache__, and narrow the blanket *.png
ignore that would have swallowed design assets.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

frontend/tests/utils/privateApi.ts

@@ -0,0 +1,22 @@
+// Note: the `PrivateService` is only available when generating the client
+// for local environments
+import { OpenAPI, PrivateService } from "../../src/client"
+
+OpenAPI.BASE = `${process.env.VITE_API_URL}`
+
+export const createUser = async ({
+  email,
+  password,
+}: {
+  email: string
+  password: string
+}) => {
+  return await PrivateService.createUser({
+    requestBody: {
+      email,
+      password,
+      is_verified: true,
+      full_name: "Test User",
+    },
+  })
+}