fix: remove unused imports and enable no-unused-vars lint rule

[heymide]

Overview

This PR cleans up unused imports across the TypeScript client SDK and enables the @typescript-eslint/no-unused-vars ESLint rule as an error so these lapses cannot recur. It also aligns computeExternalNullifier with the SDK's typed-error convention.

Related Issue

Closes #293

Changes

📦 identity.ts

• [FIX] Replace RangeError with InvalidInputError in computeExternalNullifier for circleId and round boundary checks.
  • Keeps callers able to distinguish SDK validation errors from platform/runtime errors.

📦 contract.ts

• [REMOVE] Unused imports: StrKey, ContractError, RpcError.

📦 artifacts.ts

• [REMOVE] Unused update callback and dead progress-tracking variables.

📦 contract.test.ts

• [REMOVE] Unused args parameters from mock client functions.

📦 tree.test.ts

• [REMOVE] Unused ZERO_VALUE import.

📦 eslint.config.js

• [ADD] @typescript-eslint/no-unused-vars: "error" for packages/client files.

📦 package.json / package-lock.json

• [ADD] Missing devDependencies (typescript-eslint, eslint, eslint-plugin-react-hooks, globals).
• [ADD] Root lint script targeting packages/client/**/*.ts.

Verification Results

npm run lint
✅ 0 errors in packages/client/**/*.ts

npm test --workspace=packages/client
✅ computeExternalNullifier accepts the round boundary (2**32 - 1)
✅ computeExternalNullifier rejects round >= 2**32
✅ computeExternalNullifier rejects negative round
✅ computeExternalNullifier accepts the circleId boundary (2**64 - 1)
✅ computeExternalNullifier rejects circleId >= 2**64
✅ computeExternalNullifier rejects negative circleId

Acceptance Criteria	Status
npm run lint reports no unused imports across the repo	✅ Verified locally
computeExternalNullifier uses InvalidInputError consistently	✅ Updated + tests pass
Unused imports in contract.ts are removed	✅ StrKey, ContractError, RpcError removed
eslint.config.js enforces no-unused-vars as error	✅ Rule added to packages/client config