fix: bind restores to previewed scope (#6670)

[atomantic]

Summary

• bind each accepted file-restore preview to the normalized snapshot and subdirectory request that produced it
• invalidate preview results when the scope changes and ignore late responses from obsolete preview requests
• submit live restores from the accepted preview identity and lock restore controls while the write is running

Tests

• cd client && npm test -- BackupWidget.test.jsx (13 tests)
• cd client && npm run lint
• cd client && npm run build

Review

• local code review: clean
• optional Claude review: capped at the configured 1/1 round; both findings addressed and verified

Closes #6670