feat: support fixed and percentage contributions

[None-Feeling]

Fixes #23.

Summary

• add a ContributionType enum with fixed and percentage modes
• add create_percentage_group and set_member_base_amount so members can declare the base amount used by percentage contributions
• calculate each member's contribution dynamically in percentage mode and keep RoundInfo.total_contributed as the actual pot size
• cover percentage contribution behavior with a unit test while preserving existing fixed contribution behavior

Verification

• cargo fmt -- --check
• cargo test
• cargo build --target wasm32-unknown-unknown --release
• cargo clippy -- -D warnings