feat: add approval threshold to GroupTreasuryContract initialization …

[samieazubike]

…and implement related tests

Description

Add approval threshold + withdraw-proposal storage to group_treasury (closes #121)

Scaffolds the storage and initialize changes for the upcoming multisig withdrawal flow.

• storage.rs: Added DataKey::Threshold, DataKey::ProposalCount, and DataKey::Proposal(u32) variants; a ProposalStatus enum (Active, Passed, Rejected, Executed, mirroring the proposals contract); and a WithdrawProposal struct (id, proposer, to, token, amount, approvals, rejections, status, expires_at).
• lib.rs: initialize now takes a threshold: u32 parameter - panics with "threshold must be at least 1" if threshold == 0 (validated before any storage write), persists the threshold, and seeds ProposalCount to 0. Added a get_threshold() -> u32 view function.
• test.rs: Updated existing initialize calls and added tests for the zero-threshold panic and threshold round-trip.
  Verified: cargo test -p group_treasury (20 passed) and cargo build -p group_treasury --target wasm32-unknown-unknown --release (confirms no_std compile).

Note: initialize enforces threshold ≥ 1 only - the "≤ member count" rule isn't checkable at init since the members list starts empty, so it's deferred to the proposal-lifecycle issues.

Type of change

• Bug fix
• New feature
• Documentation update
• Other

Checklist

• I have read the contributing guidelines
• I have tested my changes locally
• My code follows the project's coding standards

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