TechDraw: axonometric measurements always display in metric units

### Problem description

TechDraw axonometric measurements are always displayed in mm ignoring the selected global units system.

### Workbench affected?

TechDraw

### Steps to reproduce

Set units system to Imperial Decimal. Create a new document. Create a simple cube 1" on a side in PartDesign. Create a new sheet in TechDraw. Add a new view showing the cube in isometric view. Add an axonometric dimension to one of the edges. It displays as "25.4" The Format Spec shows just 25.4, not an actual format spec. Putting a format spec there then just displays the literal text of the format spec, not the actual dimension.

The attached axonometric.FCStd file shows the result captured in safe mode with a recorded macro "axonometric units" recording the steps.

[axonometric.FCStd.zip](https://github.com/user-attachments/files/28770137/axonometric.FCStd.zip)

### Expected behavior

Dimensions should be shown in the chosen global units system, and it should be possible to set the Format Specifier as for other measurements

### Actual behavior

Dimension always shows in mm

### Development version About Info (in Safe Mode)

```shell
OS: macOS 15.7.7
Architecture: x86_64
Version: 1.1.1.20260414 (Git shallow)
Build date: 2026/04/14 22:09:59
Build type: Release
Branch: (HEAD detached at 0108fd4b4)
Hash: 0108fd4b4850cc46e625b60e53cea7a7bbe69f8d
Python 3.11.14, Qt 6.8.3, Coin 4.0.3, Vtk 9.3.1, boost 1_86, Eigen3 3.4.0, PySide 6.8.3
shiboken 6.8.3, xerces-c 3.3.0, IfcOpenShell 0.8.4, OCC 7.8.1
Locale: English/United States (en_US)
Navigation Style/Orbit Style/Rotation Mode: CAD/Rounded Arcball/Window center
Stylesheet/Theme/QtStyle: FreeCAD.qss/FreeCAD Light/
Logical DPI/Physical DPI/Pixel Ratio: 72/109/1
```

### Last known good version (optional)

```shell

```