Guard zero-token governance vote percentages

[giwaov]

Summary

• Add a shared vote percentage helper that returns zero when total voting power is zero.
• Use the helper for account and validator poll result percentages.
• Add coverage for poll results when all account voting power is staked or in the DAO pool.

Why

Governance poll result rendering can encounter a voting group with zero total tokens. The previous direct division by total tokens could produce invalid percentage values for that group.

Testing

• git diff --check
• Not run: Go toolchain is not installed in this environment.