Fix: TS2322 Type 'X' is not assignable to type 'Y'
How to fix TypeScript error TS2322 'Type is not assignable to type'. Covers literal types vs general types, string vs String, union types, interface compatibility, generic constraints, readonly arrays, excess property checking, discriminated unions, type assertions, type widening and narrowing, React event handlers, Promise return types, and enum mismatches.