[Giveaway] Replace public distribution with winner claim lifecycle

[3m1n3nc3]

Description

The README describes a winner claim flow, but the current contract exposes distribute_prize() and sends the prize directly once the giveaway becomes claimable. Add an explicit winner-driven claim flow with clearer lifecycle semantics.

User Story

As a Winner,
I want to claim my prize explicitly,
so that giveaway completion matches the product flow and supports future claim windows or proof steps.

Requirements and Context

• Files / refs: contracts/geev-core/src/giveaway.rs, contracts/geev-core/src/test.rs, contracts/README.md
• Add claim_prize behavior that requires winner authentication
• Distinguish winner selection from prize claiming in status transitions
• Migrate or replace the current distribute_prize() path with explicit claim semantics
• Preserve fee accounting and reputation updates

Suggested Implementation

1. Keep pick_winner focused on selecting the winner and setting Claimable state
2. Add claim_prize(winner, giveaway_id)
3. Require winner auth before transferring net prize
4. Mark giveaway Completed only after successful claim

Acceptance Criteria

• Only the selected winner can claim the prize
• Winner selection and payout are separate contract actions
• Fee accounting and reputation updates still occur on successful payout

Submission Guidelines

• Branch: feat/giveaway-claim-flow
• Depends on: None
• PR: feat(giveaway): add explicit winner claim lifecycle

[mxrtins04]

Hi maintainer, can i be assigned this issue pleasee?