On-chain configurable DAO proposal thresholds

[nanaf6203-bit]

Problem Statement

Quorum & approval thresholds hardcoded.

Why it matters

Adaptive governance.

Technical Context

Add admin-setable constants.

Expected Outcome

Configurable.

Acceptance Criteria

1. Tests.
2. Kani harness.
3. Docs.

Implementation Notes

New setter.

Files or modules likely to be affected

contracts/governance/src/lib.rs.

Dependencies

None.

Difficulty level

MEDIUM.

Estimated effort

S (~1 day).