[Refactor] Refactor group state reads into a GroupRepository abstraction

[Xoulomon]

Description:
Extract all direct env.storage() calls related to group state into a GroupRepository struct to improve testability and reduce duplication.

Tasks:

• Create repository.rs with GroupRepository struct wrapping env.storage() calls
• Migrate all storage reads/writes in group.rs and payout.rs to use the repository
• Verify all existing tests pass without modification after the refactor

Difficulty: medium
Points: 150

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[Daveside9]

@Daveside9 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

can i do this? merge in 2hours.

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[felixkamau]

@felixkamau has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi I would like to work and contribute on this

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[knytcomics-ui]

@knytcomics-ui has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

gm i would like to apply to this issue

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