[Bug]: Tighten magic-link verification lifecycle cleanup

[Bccorb]

What happened?

The magic-link verification screen creates async resources inside an effect but does not fully clean them up:

• BroadcastChannel
• redirect setTimeout

This is a small but worthwhile hardening task.

Current behavior

In src/views/VerifyMagicLink.tsx:

• a BroadcastChannel is created after successful verification
• a timeout is scheduled to redirect the user
• neither resource is explicitly cleaned up on unmount

Steps to Reproduce

Send a magic link, click the link to verify, not the broadcast is still mounted.

Expected Behavior

Acceptance criteria

• No unclosed BroadcastChannel remains after component unmount
• Redirect timeout is cleared on unmount
• Success flow still posts the success message and redirects correctly
• Error and missing-token cases still behave correctly

Environment

No response

Logs / stack traces