fix(GcodePreview): reset only null coordinates

[pedrolamas]

Previously, the toolhead coordinates were checked and reset individually, but changes from 4e6f7aa caused this issue as the move object was copied completely, overriding previous values - this reverts back to the original approach, fixing the problem.

Fixes #1774

[Copilot]

Pull request overview

This PR fixes a regression in the GcodePreview toolhead position calculation. The previous Object.assign approach would override previously found non-null coordinates with null values from subsequent moves. This fix reverts to individually checking and assigning only null coordinates.

• Replaces Object.assign with individual coordinate null checks
• Adds a count variable to track how many coordinates have been found
• Only assigns coordinates when output is NaN and move coordinate is non-null

[Copilot]

The loop termination condition using count < 3 is incorrect. The loop should continue while any coordinate is still NaN, not just until 3 coordinates are found. If all three coordinates (x, y, z) are found from a single move, the count will be 3 but the loop will still iterate through all remaining moves unnecessarily. The condition should be count < 3 && (Number.isNaN(output.x) || Number.isNaN(output.y) || Number.isNaN(output.z)) to match the original logic and stop early when all coordinates are found.

Suggested change

	for (let i = moveIndex, count = 0; i >= 0 && count < 3; i--) {
	for (
	let i = moveIndex, count = 0;
	i >= 0 && count < 3 && (Number.isNaN(output.x) || Number.isNaN(output.y) || Number.isNaN(output.z));
	i--
	) {

[pedrolamas]

Copilot is wrong: count increases for every coordinate that is currently NaN and that is set to a non-null value, which should happen 3 times, hence the check.

Keep code as is.