feat: add smooth focus transition animation for opacity and border

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.

src/mango.c

@@ -161,7 +161,7 @@ enum {
 }; /* EWMH atoms */
 #endif
 enum { UP, DOWN, LEFT, RIGHT, UNDIR }; /* smartmovewin */
-enum { NONE, OPEN, MOVE, CLOSE, TAG };
+enum { NONE, OPEN, MOVE, CLOSE, TAG, FOCUS };
 enum { UNFOLD, FOLD, INVALIDFOLD };
 enum { PREV, NEXT };
 
@@ -339,6 +339,12 @@ struct Client {
 	int isunglobal;
 	float focused_opacity;
 	float unfocused_opacity;
+	float current_opacity;
+	float target_opacity;
+	unsigned int opacity_animation_frames;
+	unsigned int opacity_animation_passed;
+	float current_border_color[4];
+	float target_border_color[4];
 	char oldmonname[128];
 	int noblur;
 	double master_mfact_per, master_inner_per, stack_innder_per;
@@ -777,6 +783,7 @@ struct dvec2 *baked_points_move;
 struct dvec2 *baked_points_open;
 struct dvec2 *baked_points_tag;
 struct dvec2 *baked_points_close;
+struct dvec2 *baked_points_focus;
 
 static struct wl_event_source *hide_source;
 static bool cursor_hidden = false;
@@ -3100,7 +3107,13 @@ void focusclient(Client *c, int lift) {
 
 		// change border color
 		c->isurgent = 0;
-		setborder_color(c);
+		// Start border color animation to focused
+		memcpy(c->target_border_color, focuscolor, sizeof(c->target_border_color));
+
+		// Start opacity animation to focused
+		c->target_opacity = c->focused_opacity;
+		c->opacity_animation_frames = (animation_duration_focus * 60) / 1000; // 60fps
+		c->opacity_animation_passed = 0;
 	}
 
 	if (c && !c->iskilling && c->foreign_toplevel)
@@ -3128,7 +3141,13 @@ void focusclient(Client *c, int lift) {
 			 * probably other clients */
 		} else if (w && !client_is_unmanaged(w) &&
 				   (!c || !client_wants_focus(c))) {
-			setborder_color(w);
+			// Start border color animation to unfocused
+			memcpy(w->target_border_color, bordercolor, sizeof(w->target_border_color));
+
+			// Start opacity animation to unfocused
+			w->target_opacity = w->unfocused_opacity;
+			w->opacity_animation_frames = (animation_duration_focus * 60) / 1000; // 60fps
+			w->opacity_animation_passed = 0;
 
 			client_activate_surface(old_keyboard_focus_surface, 0);
 		}
@@ -3512,6 +3531,12 @@ void init_client_properties(Client *c) {
 	c->fake_no_border = false;
 	c->focused_opacity = focused_opacity;
 	c->unfocused_opacity = unfocused_opacity;
+	c->current_opacity = unfocused_opacity;
+	c->target_opacity = unfocused_opacity;
+	c->opacity_animation_frames = 0;
+	c->opacity_animation_passed = 0;
+	memcpy(c->current_border_color, bordercolor, sizeof(c->current_border_color));
+	memcpy(c->target_border_color, bordercolor, sizeof(c->target_border_color));
 	c->nofadein = 0;
 	c->nofadeout = 0;
 	c->no_force_center = 0;