Add pad-extend option

Adds a new boolean option `pad-extend` that extends edge cell
background colors into the terminal padding/margins, similar to
Ghostty's behavior. When enabled, the left/right margins use the
background color of the adjacent edge cells, and top/bottom margins
use the colors of the first/last row cells.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

config.h

@@ -239,6 +239,7 @@ struct config {
     unsigned pad_top;
     unsigned pad_right;
     unsigned pad_bottom;
+    bool pad_extend;
     enum center_when center_when;
 
     bool resize_by_cells;