feat: add multi-token group contributions

[TUPM96]

Closes #4

/claim #4

Summary

• Add multi-token group creation with an accepted-token allowlist while keeping the existing single-token create_group path backward-compatible.
• Add contribute_with_token(member, group_id, token) with allowlist validation and per-token round accounting.
• Distribute each token pot to the round recipient when a multi-token round completes.
• Add regression tests for default single-token storage, empty allowlist rejection, accepted multi-token contribution/payout flow, and unaccepted token rejection.

Validation

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

Payout details can be provided privately on acceptance.

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