Add maximum group duration limit

[WuXieSec]

This PR enforces a maximum total duration for savings groups to prevent indefinitely long cycles, as requested in #55.

Changes:

• Add MAX_GROUP_DURATION constant (365 days in seconds)
• Validate total_rounds * cycle_length <= MAX_GROUP_DURATION during start_group
• Add DurationTooLong error variant to ContractError
• Return clear error if duration exceeds limit
• Add test coverage for both valid and invalid durations

Why this matters:
Without a duration limit, groups could theoretically run for years or decades, which:

• Increases risk for participants
• Makes planning difficult
• Could lead to abandoned groups
• Complicates dispute resolution

Implementation:
The validation happens in start_group (not create_group) because total_rounds equals the final member count, which isn't known until the group starts.

Example:

• 10 members × 30-day cycles = 300 days total ✅
• 10 members × 50-day cycles = 500 days total ❌ (exceeds 365-day limit)

Closes #55