Fix/vote optimistic rollback

[Yentec]

Description

Fix optimistic vote rollback on action failure

Changes

• Added router.refresh() in the error branch of VoteButton.handleClick — useOptimistic does not roll back automatically; router.refresh() triggers a server re-render that resets the optimistic state to the real value

Validation

• Clean typecheck
• Build successful
• Existing unit tests pass (server/actions/tests/votes.test.ts)
• Manually verified: failed vote (e.g. rate-limited) reverts the UI count

Closes #14