Option to disable the bar double-click transparency toggle

[itchyfeetleech]

Double-left-clicking empty bar space toggles bar transparency (shell/plugins/bar/Bar.qml, CenterGestureArea.onDoubleClicked -> toggleTransparency(), persisted to bar.transparent in shell.json). It's easy to trigger accidentally, and there is no way to turn the gesture off for users who always want an opaque bar.

Suggestion: a bar config key, e.g. "transparencyToggle" defaulting to true, read in applyBarConfig and honored by the double-click handler:

{ "bar": { "transparent": false, "transparencyToggle": false } }

With it set to false, double-clicks on empty bar space do nothing and the bar stays as configured. Default true keeps current behavior, including for existing user configs (which have no deep-merge, so the default must live in code).

Happy to PR this (Bar.qml + shell README shape docs) if maintainers like the shape of it.