This adds animated transitions when switching focus between windows.
Both window opacity and border color now fade smoothly using cubic
bezier easing instead of changing instantly.
Implementation:
- Added animation_duration_focus config option (default 400ms)
- Added animation_curve_focus for cubic bezier easing curve
- Window opacity and border color animate together when focus changes
- Uses existing animation infrastructure (baked bezier points)
The feature is backwards compatible and can be disabled by setting
animation_duration_focus=0 in config file.
Changes affect 5 files with minimal additions to keep code clean.
