Implement round deadline extension by admin vote

[big14way]

Allow group admin to extend the current round deadline when members need more time to contribute.

Acceptance Criteria:

• Add extend_deadline(admin, group_id, extra_seconds) function
• Validate admin authorization
• Cap maximum extension at one cycle_length
• Maximum 2 extensions per round
• Emit deadline_extended event

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

[er1c-cartman]

Submitted a focused implementation for this in PR #146.

What it covers:

• extend_deadline(admin, group_id, extra_seconds) exposed through the contract client
• group/protocol admin authorization
• active-group validation
• max extension of one cycle_length per call
• max two extensions per round via RoundInfo.deadline_extensions
• dl_ext event emission
• regression coverage for deadline updates, extension count, third-extension rejection, non-admin rejection, and oversized extension rejection

Validation:

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