fix(ui): keep permission dialog responsive between sequential prompts

[DmitriyKost]

Summary

Sequential permission prompts could leave the UI stale: after selecting a permission, the permission display could be refreshed while _processing was still true, causing the next queued permission to skip rendering until another keypress forced a refresh.

• Reorder on_select: clear _processing before remove_permission so the next render is not skipped.
• Swap the 50ms vim.defer_fn for vim.schedule, and add a reentrancy guard at the top of on_select.
• Move the post-removal render into remove_permission itself, and drop the now-redundant render branch in on_permission_replied.

Test plan

• ./run_tests.sh -t tests/unit/permission_window_spec.lua - 25/25 pass
• ./run_tests.sh -t tests/unit/permission_integration_spec.lua - 15/15 pass
• Manually reproduced the stale UI on 6ab7d2f; gone on this branch

[sudo-tee]

Thanks for the fix.

Works pretty well