fix(auth): bind OAuth state to the initiating browser

[albertovincenzi]

Problem

The OAuth state JWT was signed, but it was not tied to the browser that initiated the login. Signing prevents mutation; it does not prevent an attacker from initiating and completing their own Google login, then sending that valid callback URL to a victim. Loading it would replace the victim’s Gate session with the attacker’s identity (login CSRF / session swapping).

Fix

• set the exact signed state in a five-minute, HttpOnly, SameSite=Lax cookie scoped to the callback path
• require an exact cookie/query-state match before exchanging the authorization code
• add Secure on HTTPS deployments
• delete the transaction cookie after a successful callback
• reuse one cookie parser for session and OAuth cookies
• add regression tests for missing/mismatched state and cookie attributes

Notes for Alice

This PR is intentionally stacked on #15 because both touch the OAuth state flow; review/merge #15 first. SameSite=Lax still allows Google’s top-level GET redirect, while the host-only callback-scoped cookie prevents a callback prepared in another browser from being accepted. No provider configuration changes are required.

Verification

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings

[alice-viola]

Landed on master via #67 (merge commit 944ee9b) as part of the 62-PR integration — this PR's head commit a9915c1 is an ancestor of master. GitHub could not mark it merged automatically because its base is fix/oauth-next-open-redirect, not master. Closing as landed.