Codex 起手式四關卡：定義任務 → 審計畫 → 核 diff → 驗證才算完

GATE 1 — FRAME Goal: After login, redirect the user back to the page they came from instead of always /dashboard. Done when: visiting /watchlist while logged out, logging in, lands back on /watchlist; existing /dashboard default still works when there's no return path; `npx tsc --noEmit` passes. Open questions: Should the return_to be validated against an allowlist to avoid open-redirect? (recommend yes) — no other blockers. [STOP — awaiting your confirmation] GATE 2 — PLAN 1. middleware.ts: capture original path into a signed `return_to` cookie on the redirect-to-login. 2. app/login/route.ts: after successful auth, read return_to, validate it's a same-origin relative path, else fall back to /dashboard. 3. Add test in __tests__/login-redirect.test.ts covering: valid return, missing return, open-redirect attempt. Risks: open-redirect if return_to isn't validated; cookie must be httpOnly + same-site. [STOP — awaiting plan approval]