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.

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

src/UICircle.cpp

@@ -360,7 +360,7 @@ PyGetSetDef UICircle::getsetters[] = {
      "Outline color of the circle", NULL},
     {"outline", (getter)UICircle::get_outline, (setter)UICircle::set_outline,
      "Outline thickness (0 for no outline)", NULL},
-    {"click", (getter)UIDrawable::get_click, (setter)UIDrawable::set_click,
+    {"on_click", (getter)UIDrawable::get_click, (setter)UIDrawable::set_click,
      "Callable executed when circle is clicked.", (void*)PyObjectsEnum::UICIRCLE},
     {"z_index", (getter)UIDrawable::get_int, (setter)UIDrawable::set_int,
      "Z-order for rendering (lower values rendered first).", (void*)PyObjectsEnum::UICIRCLE},