fix(polls): give composer errors and checkbox options room to breathe

[adityapradhan10]

Summary

• Add mt-2 on the discussion composer ErrorMessage, so a poll (or comment) validation error is no longer flush against Discard / Submit.
• Group the Anonymous and Multiple answers checkboxes with flex gap-2 so they aren't cramped.

Fixes #551

Test plan

• Open a discussion, switch the composer to Poll, submit with duplicate option text, and confirm a gap between the buttons and ValidationError: Duplicate options not allowed.
• Confirm Anonymous and Multiple answers sit side by side with a small gap, and that checking one still disables the other.
• Trigger a comment insert error and confirm it also has spacing below the action row.

Evidence

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

The changes are presentation-only and preserve existing bindings, disabled states, and update handlers without an actionable regression.

_{Reviews (1): Last reviewed commit: "fix(polls): give composer errors and che..." | Re-trigger Greptile}