Add flexible contribution models

[Scottcjn]

Summary

• add a ContributionType enum so groups can use fixed or percentage-based contributions
• store per-member base amounts for percentage groups and require them before the group starts
• calculate round contributions dynamically from each member's declared base amount and keep payout totals driven by the actual contributed pot
• cover the new flow with a percentage-mode unit test while keeping the existing fixed-mode API intact

Validation

• cargo test -q
• git diff --check

Closes #23.