Cursor location is calculated incorrectly for DevTool themes

[celer]

For DevTools Dark/Light the cursor location in the LineEdit widget is calculated incorrectly

To reproduce in the gallery app:

1. Select DevTools Dark theme
2. click to enter text in the TextField, press and hold the 'd' key
3. The cusor does not end up at the end of the text, it isn't shited to the right at the correct interval:

[kolkov]

Thanks for the clear reproduction steps @celer!

Root cause identified: The DevTools, Fluent, and Cupertino theme painters calculate cursor position using a hardcoded approximation (charWidth = fontSize * 0.55), while the Material3 and Default painters correctly use canvas.MeasureText() for accurate text measurement.

This was introduced when the DevTools theme was created by copy-pasting from older code that predated the MeasureText fix.

Affected themes:

• DevTools (13pt JetBrains Mono — most visible due to monospace font)
• Fluent (14pt Inter — less noticeable)
• Cupertino (15pt — less noticeable)

Fix: Replace charWidthRatio approximation with canvas.MeasureText() in all 3 themes (6 locations). Same pattern already working correctly in Material3.

Will fix in the next release.