Use const and let, never var

Always use `const` by default, `let` when reassignment is needed:
- `const`: Block-scoped, cannot be reassigned
- `let`: Block-scoped, can be reassigned
- `var`: Function-scoped, hoisted - NEVER USE

```typescript
// Good
const users = await fetchUsers();
let count = 0;
for (const user of users) {
count++;
}

// Bad
var users = await fetchUsers();
```