refactor: Rename click kwarg to on_click for API consistency (closes #126)

BREAKING CHANGE: Constructor keyword argument renamed from `click` to
`on_click` for all UIDrawable types (Frame, Caption, Sprite, Grid, Line,
Circle, Arc).

Before: Frame(pos=(0,0), size=(100,100), click=handler)
After:  Frame(pos=(0,0), size=(100,100), on_click=handler)

The property name was already `on_click` - this makes the constructor
kwarg match, completing the callback naming standardization from #139.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/UICaption.cpp

@@ -331,7 +331,7 @@ int UICaption::init(PyUICaptionObject* self, PyObject* args, PyObject* kwds)
     static const char* kwlist[] = {
         "pos", "font", "text",  // Positional args (as per spec)
         // Keyword-only args
-        "fill_color", "outline_color", "outline", "font_size", "click",
+        "fill_color", "outline_color", "outline", "font_size", "on_click",
         "visible", "opacity", "z_index", "name", "x", "y",
         nullptr
     };