feat: add percentage contribution groups

[TUPM96]

/claim #23

Summary

• add ContributionType::{Fixed, Percentage} to group state
• keep existing create_group behavior as fixed-amount groups
• add create_percentage_group for percentage-of-base contributions
• let members declare base amounts with set_member_base_amount
• calculate per-member required contributions dynamically during contribute
• validate percentage groups before start so every member has a usable base amount
• add views for base amount and required contribution
• add tests for invalid percentage config, missing base amounts, and dynamic pot sizing

Validation

• cargo fmt --check
• cargo test --package sorosave
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check

Payout details can be provided privately on acceptance.

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