Add input validation for contribution amount precision

Ensure contribution amounts align with the token's decimal precision.

**Acceptance Criteria:**
- Query token decimals during `create_group`
- Validate contribution_amount is a whole number of minimum token units
- Reject amounts that would cause rounding issues
- Store token decimals in group metadata

**Files:** `contracts/sorosave/src/group.rs`, `contracts/sorosave/src/types.rs`