Preserve valid screen coordinates at the top and left edges

Several coordinate guards treat zero as missing even though (0, y), (x, 0), and (0, 0) are valid screen positions. The behavior appears in apps/ui-tars set-of-marks and remote drag handling, packages/ui-tars operators, and the browser operator implementations. Please preserve finite zero coordinates while still rejecting null or malformed input, with focused regression coverage for edge coordinates.