desktopchangeosd: add configurable font size

Brainstorm
1

The Desktop Change OSD script currently displays the desktop name using a Kirigami.Heading with no configurable font size. On high-resolution or large displays, the text can be too small to be useful.

This patch adds a FontSize configuration key (default: current implicit size) readable via KWin.readConfig("FontSize", default), consistent with the existing PopupHideDelay and TextOnly keys already present in the script.

Changes:

  • osd.qml: read FontSize from config and apply to Kirigami.Heading

  • contents/config/main.xml: declare the FontSize entry

  • contents/ui/config.ui: add a spin box in the script settings panel

No breaking changes — the default value preserves current behavior.