fix(crowdfund): block refund_single once milestone mode is active

[chideraisiguzor]

closes #1373

Description

Related Issues

Closes #

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• CI / Infrastructure

Required CI Checks

The following status checks must pass before this PR can be merged:

Frontend CI (frontend_ci.yml — ci-frontend job):

• TypeScript type-check (pnpm --filter @stellar-raise/frontend typecheck)
• Vitest test suite (pnpm --filter @stellar-raise/frontend test)
• ESLint across all workspaces (pnpm lint)
• Prettier format check (pnpm format:check)

Rust CI (rust_ci.yml — check job):

• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo build --release --target wasm32-unknown-unknown
• cargo test --workspace

Checklist

• My branch is based off develop, not main
• I have run cargo fmt --all and the code is properly formatted
• I have run cargo clippy --all-targets -- -D warnings with no warnings
• I have run cargo test and all tests pass
• I have run pnpm typecheck with no type errors
• I have run pnpm test and all frontend tests pass
• I have run pnpm lint with no ESLint errors
• I have run pnpm format:check with no formatting violations
• I have added tests for any new functionality
• All public functions have /// doc comments
• I have updated CHANGELOG.md if applicable
• My commit messages follow the conventional commits format

Screenshots / Logs (if applicable)

Additional Notes