Fix: Handle edge case when all members leave during Forming

[WuXieSec]

This PR fixes the edge case where all non-admin members leave a group during the Forming state, as requested in #22.

Changes:

• Add delete_group(admin, group_id) function
• Only works in Forming state with 1 member (admin)
• Clean up all storage keys (group data and member index)
• Add remove_group helper in storage.rs
• Add comprehensive test coverage

Why this matters:
Without this fix, if all members leave a group during Forming:

• The admin is stuck with an unusable group
• Storage is wasted on orphaned groups
• The admin can't clean up and start fresh

Implementation:
The delete_group function:

• Requires admin authorization
• Only works in Forming state
• Only works when exactly 1 member remains (the admin)
• Cleans up both the group data and member index

Test coverage:

• ✅ Delete empty group (only admin remains)
• ❌ Delete group with multiple members → InsufficientMembers
• ❌ Non-admin tries to delete → Unauthorized
• ❌ Delete active group → GroupNotForming

Closes #22