Enable TypeScript strict mode in frontend

[SL-Mar]

Problem

Frontend tsconfig.json may not have strict mode fully enabled, allowing implicit any types and unchecked nulls. This reduces type safety and makes refactoring riskier.

Steps to fix

1. Set "strict": true in tsconfig.json
2. Enable noImplicitAny, strictNullChecks, strictFunctionTypes
3. Fix type errors that emerge
4. Consider adding pre-commit ESLint hook

Files

• frontend/tsconfig.json
• Various .tsx files (fix type errors)