demo: redirect OAuth errors back to originating flow page

[aspiers]

Problem

The OAuth callback (/api/oauth/callback) always redirects errors to /?error=auth_failed, regardless of which flow page initiated the login. This means the error banner in FlowLoginPage is never shown on Flow 2/3/4 — users always land on Flow 1 with the error.

Solution

Thread the originating flow path through the OAuth state cookie (or as a separate return_to query param on the auth redirect URL), so the callback can redirect errors back to the correct page:

• Flow 2 error → /flow2?error=auth_failed
• Flow 3 error → /flow3?error=auth_failed
• etc.

Notes

• Happy path is unaffected — all flows work correctly on success
• Only affects the demo app's error UX
• Identified during review of PR [HYPER-192] add flows 3/4 to test picker-with-random handle mode #33