refactor: Rename click property to on_click (closes #139)

Breaking change: callback property standardized to on_* pattern.
- `drawable.click` → `drawable.on_click`

Updated all C++ bindings (8 files) and Python test usages.
Note: src/scripts changes tracked separately (in .gitignore).

This establishes the naming pattern for future callbacks:
on_click, on_enter, on_exit, on_move, on_key, etc.

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

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

src/UILine.cpp

@@ -422,8 +422,8 @@ PyGetSetDef UILine::getsetters[] = {
      MCRF_PROPERTY(color, "Line color as a Color object."), NULL},
     {"thickness", (getter)UILine::get_thickness, (setter)UILine::set_thickness,
      MCRF_PROPERTY(thickness, "Line thickness in pixels."), NULL},
-    {"click", (getter)UIDrawable::get_click, (setter)UIDrawable::set_click,
-     MCRF_PROPERTY(click, "Callable executed when line is clicked."),
+    {"on_click", (getter)UIDrawable::get_click, (setter)UIDrawable::set_click,
+     MCRF_PROPERTY(on_click, "Callable executed when line is clicked."),
      (void*)PyObjectsEnum::UILINE},
     {"z_index", (getter)UIDrawable::get_int, (setter)UIDrawable::set_int,
      MCRF_PROPERTY(z_index, "Z-order for rendering (lower values rendered first)."),