fix(auth): return to origin path after GitHub login

Sign-in saves the current path in sessionStorage so the OAuth callback
can route back instead of always landing on Home.

src/modules/user/service/oauthReturnPath.ts

@@ -0,0 +1,7 @@
+export const GITHUB_OAUTH_RETURN_PATH_KEY = "github-oauth-return-path"
+
+export const consumeGithubOAuthReturnPath = (): string | null => {
+  const path = sessionStorage.getItem(GITHUB_OAUTH_RETURN_PATH_KEY)
+  sessionStorage.removeItem(GITHUB_OAUTH_RETURN_PATH_KEY)
+  return path
+}