Add contribution amount flexibility (fixed vs percentage)

[big14way]

Support both fixed amount and percentage-of-income contribution models.

Acceptance Criteria:

• Add ContributionType enum (Fixed, Percentage)
• For percentage mode, members declare their base amount
• Calculate pot size dynamically

Files: contracts/sorosave/src/types.rs, contracts/sorosave/src/contribution.rs

[Scottcjn]

Opened a PR for this here: #88

Implemented ContributionType support with a backward-compatible fixed mode, added member-declared base amounts for percentage groups, and verified the dynamic pot path with cargo test -q.