Feat/api integration layer

[mijinummi]

Objective

• Create centralized API client with consistent error handling, interceptors, and type safety.

Changes

• Added client.ts with base Axios config and SSR guard.
• Added interceptors.ts for JWT attachment, 401 handling, and retry logic.
• Added error-handler.ts for standardized error responses.
• Added types.ts for common API response and error interfaces.

Acceptance Criteria

• ✅ Base client configured with env BASE_URL.
• ✅ JWT auto-attached.
• ✅ 401 redirects to login.
• ✅ Errors standardized.
• ✅ Retry logic implemented.
• ✅ Type safety via generics.
• ✅ No API calls during SSR.

Closes #245