fix(website): prevent revoke modal from closing on backdrop click (#6402)

## Summary

Fixes #6401.

The sequence revoke confirmation modal (shown when clicking "Revoke this
sequence" on the sequence details page) was dismissable by clicking the
backdrop. This is easy to do accidentally — and discards whatever
revocation reason the user has typed — so the modal felt fragile and
confusing.

This PR flips `closeOnClickOutside` from `true` to `false` in
`displayRevocationDialog`
(`website/src/components/SequenceDetailsPage/RevokeButton.tsx`). The
modal now only closes via the explicit Cancel button, the ✕ button in
the corner, or Confirm.

This matches the behavior users typically expect from a
destructive-action confirmation modal: an outside click is much more
likely to be accidental than intentional, especially when there is form
state inside.

## Test

Theo checked working as intended

🤖 Generated with [Claude Code](https://claude.com/claude-code)

🚀 Preview: https://fix-revoke-modal-backdrop.loculus.org

Co-authored-by: theosanderson-agent <theo@theo.io>
Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Co-authored-by: Theo Sanderson <theo@sndrsn.co.uk>

Author: 3 people

website/src/components/SequenceDetailsPage/RevokeButton.tsx

@@ -69,7 +69,7 @@ interface DisplayRevocationProps {
 
 export const displayRevocationDialog = ({ dialogText, onConfirmation }: DisplayRevocationProps) => {
     confirmAlert({
-        closeOnClickOutside: true,
+        closeOnClickOutside: false,
 
         customUI: ({ onClose }) => (
             <RevocationDialog