[SC-17] Implement claim_dividends function

[yusuftomilola]

Overview

Implement the dividend claim function that lets token holders withdraw their share inside contracts/contrib/.

Location

All work must be inside contracts/contrib/src/dividends.rs.

Acceptance Criteria

• claim_dividends(env, asset_id, holder) — requires auth from holder; reads unclaimed_dividends from OwnershipRecord; panics if amount is 0; resets unclaimed_dividends to 0; emits claim event with amount
• get_total_distributed(env, asset_id) -> i128 — returns total dividends distributed to date
• Unit tests: claim with zero balance panics, successful claim resets balance