Fix caret behavior on input field activation

[pvcraven]

Summary

Fix UIInputText caret not activating when switching between input fields.

Description

_on_active_changed only handled deactivation (_active → False), ignoring activation (_active → True). This meant caret.on_activate() was never called through the Property callback path.

The normal activation path is: click → on_click → activate(). But UIInteractiveWidget.on_event calls _grap_active() on mouse press, setting widget._active = True before on_click fires on mouse release. When activate() runs, its if self._active: return guard bails out immediately — caret.on_activate() is never reached.

The caret appears to work on first use because pyglet's Caret constructor defaults _active = True. After the first deactivate() sets caret._active = False, no code path restores it.

The fix adds the activation case to _on_active_changed, so the caret is properly activated whenever the widget's _active property becomes True, regardless of whether it was set by activate() or _grap_active().

Test plan

• Place two or more UIInputText widgets on screen
• Click between them repeatedly — caret should remain visible and blinking in the focused field