Fix transform gizmo local translation snapping (#24486)

# Objective

Fixes #24431.

When translation snapping is enabled, the transform gizmo currently
snaps world-space position components with `snap_axis`. This works for
world-aligned axes, but breaks local-space translation on rotated
entities because a local axis can affect multiple world-space
components.

## Solution

Snap the scalar drag distance along the selected gizmo axis instead of
snapping `x/y/z` position components.
This keeps snapping aligned with the actual drag axis in both world and
local space.

## Testing

- `cargo check` in a local example using `TransformGizmoPlugin`
- `cargo fmt -p bevy_gizmos --check`
- I ran the reproduction actions from #24431 and verified the issue
before applying the fix: with a rotated object in local space,
translation snapping moved the object along world-space components
instead of the displayed local gizmo axis. After applying this change, I
repeated the same steps and confirmed that snapped translation follows
the selected local axis as expected.