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.

frontend/src/hooks/useAuth.ts

@@ -0,0 +1,70 @@
+import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query"
+import { useNavigate } from "@tanstack/react-router"
+
+import {
+  type Body_login_login_access_token as AccessToken,
+  LoginService,
+  type UserPublic,
+  type UserRegister,
+  UsersService,
+} from "@/client"
+import { handleError } from "@/utils"
+import useCustomToast from "./useCustomToast"
+
+const isLoggedIn = () => {
+  return localStorage.getItem("access_token") !== null
+}
+
+const useAuth = () => {
+  const navigate = useNavigate()
+  const queryClient = useQueryClient()
+  const { showErrorToast } = useCustomToast()
+
+  const { data: user } = useQuery<UserPublic | null, Error>({
+    queryKey: ["currentUser"],
+    queryFn: UsersService.readUserMe,
+    enabled: isLoggedIn(),
+  })
+
+  const signUpMutation = useMutation({
+    mutationFn: (data: UserRegister) =>
+      UsersService.registerUser({ requestBody: data }),
+    onSuccess: () => {
+      navigate({ to: "/login" })
+    },
+    onError: handleError.bind(showErrorToast),
+    onSettled: () => {
+      queryClient.invalidateQueries({ queryKey: ["users"] })
+    },
+  })
+
+  const login = async (data: AccessToken) => {
+    const response = await LoginService.loginAccessToken({
+      formData: data,
+    })
+    localStorage.setItem("access_token", response.access_token)
+  }
+
+  const loginMutation = useMutation({
+    mutationFn: login,
+    onSuccess: () => {
+      navigate({ to: "/" })
+    },
+    onError: handleError.bind(showErrorToast),
+  })
+
+  const logout = () => {
+    localStorage.removeItem("access_token")
+    navigate({ to: "/login" })
+  }
+
+  return {
+    signUpMutation,
+    loginMutation,
+    logout,
+    user,
+  }
+}
+
+export { isLoggedIn }
+export default useAuth