Implement group cloning for recurring savings circles

[big14way]

Allow admins to clone a completed group's settings to start a new cycle with the same members.

Acceptance Criteria:

• Add clone_group(admin, source_group_id) -> u64 function
• Copy token, contribution_amount, cycle_length, max_members from source
• Optionally auto-invite previous members
• New group starts in Forming status
• Reference source group in metadata

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

[jiarongmai]

I can implement this feature. I have experience with similar tasks and can deliver high-quality code. Ready to start immediately!