feat: add reusable group templates

[TUPM96]

Closes #78

/claim #78

Summary

• add a GroupTemplate contract type with the requested name, token, contribution amount, cycle length, and max member fields
• store templates per admin in persistent storage with a hard cap of 10 templates per admin
• expose save_template, get_template, and create_from_template contract methods
• reuse existing group creation validation and member indexing when creating from a template
• add regression coverage for saving, reading, creating from a template, per-admin lookup, and the 10-template limit

Validation

• cargo fmt --check
• cargo test --package sorosave (11 passed)
• cargo clippy --package sorosave --all-targets -- -D warnings
• git diff --check

Payout details can be provided privately on acceptance.

[Copilot]

Copilot encountered an error and was unable to review this pull request. You can try again by re-requesting a review.