Rename pane → window title bar per reviewer feedback

- Rename all options from pane_title_* to window_title_*
- Use foreground/background instead of fg/bg in color option names
- Change color options to to_color_or_none defaulting to None,
  falling back to corresponding tab bar colors
- Add bell_symbol, activity_symbol, progress_percent template vars
  using existing bell_on_tab and tab_activity_symbol options
- Add custom script support via window_title_bar.py in config dir
  (draw_window_title function exposed as {custom} in templates)
- Update C structs, Python references, and regenerate config files

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

tools/cmd/at/set_colors.go

@@ -15,15 +15,19 @@ import (
 var nullable_colors = map[string]bool{
 	// generated by gen-config.py do not edit
 	// NULLABLE_COLORS_START
-	"active_border_color":  true,
-	"cursor":               true,
-	"cursor_text_color":    true,
-	"cursor_trail_color":   true,
-	"selection_background": true,
-	"selection_foreground": true,
-	"tab_bar_background":   true,
-	"tab_bar_margin_color": true,
-	"visual_bell_color":    true,
+	"active_border_color":                  true,
+	"cursor":                               true,
+	"cursor_text_color":                    true,
+	"cursor_trail_color":                   true,
+	"selection_background":                 true,
+	"selection_foreground":                 true,
+	"tab_bar_background":                   true,
+	"tab_bar_margin_color":                 true,
+	"visual_bell_color":                    true,
+	"window_title_bar_active_background":   true,
+	"window_title_bar_active_foreground":   true,
+	"window_title_bar_inactive_background": true,
+	"window_title_bar_inactive_foreground": true,
 	// NULLABLE_COLORS_END
 }