TechDraw: RichTextAnnotation : Frame not centered

### Is there an existing issue for this?

- [x] I have searched the existing issues

### Problem description

Margins are not the same on all sides of the text.

### Full version info

```shell
OS: Windows 11 build 26100
Architecture: x86_64
Version: 1.1.0dev.14555 (Git shallow)
Build date: 2025/10/07 22:49:55
Build type: Release
Branch: main
Hash: 68f083a6d169ec523fa9d46b7cc40addb1e4ffbe
Python 3.11.13, 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 0.8.2, 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 Dark.qss/FreeCAD Dark/
Logical DPI/Physical DPI/Pixel Ratio: 96/88.8003/1.25
```

### Subproject(s) affected?

TechDraw

### Anything else?

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Reproduce the issue in the TechDraw workbench using a RichTextAnnotation and compare the frame margins shown in the attached image. Trace the annotation's rendering or layout entry point; done means the frame is centered with equal margins around the text.

Written by the indexing model from the issue text.