Polish: Improve visual feedback for the quadratic voting cost calculator

[Mosas2000]

Summary

The current VotingInterface shows the quadratic cost as simple text. A more visual representation would help users understand the exponential cost relationship.

Recommended Improvements

1. Add a visual cost curve showing how cost increases with weight
2. Show a slider input for vote weight with dynamic cost display
3. Display the user's remaining stake balance after the vote
4. Warn when the vote cost approaches the user's total stake
5. Show a comparison of voting power per STX spent
6. Add a tooltip explaining quadratic voting to new users

Impact

Low - Better cost visualization is educational and reduces voting errors.