fix: multiple file selection on tab navigation

[kristian-zendato]

• Resolves: [accessibility] files: cannot select multiple files with keyboard only #62250

Summary

Keyboard-only multi-select in the files list was broken: after selecting a file, focus was trapped between the “select all” checkbox and the batch actions overlay, so Tab/Shift+Tab could not reach file rows again (regression from #58457).

This removes the custom Tab/focusout focus trapping between the select-all checkbox and the first batch action button in FilesListTableHeader.vue and FilesListTableHeaderActions.vue, so focus follows the normal document order and keyboard multi-select works again.

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI