Add data type system (string, number, boolean, array, object)

**Description:**
Implement a proper type system for variables with type checking.

**Tasks:**
- [ ] Define type system
- [ ] Add type annotations to variables
- [ ] Implement type checking
- [ ] Add type conversion
- [ ] Show types in UI
- [ ] Add type validation
- [ ] Document type system

**Acceptance Criteria:**
- Types enforced
- Conversions automatic
- Errors on type mismatch
- UI shows types