Adjust screen buffer and PTY sizing for box mode

When box mode is active, reduce both the screen buffer and PTY size
by 2 in each dimension to reserve space for the border:

- window_pane_resize(): Reduce screen buffer to (sx-2, sy-2)
- window_pane_send_resize(): Report PTY size as (sx-2, sy-2)
- options_push_changes(): Resize screen/PTY when option changes

This ensures the shell receives the correct terminal dimensions and
content cannot be written in the border area.

options.c

@@ -1244,6 +1244,29 @@ options_push_changes(const char *name)
 		RB_FOREACH(w, windows, &windows)
 			layout_fix_panes(w, NULL);
 	}
+	if (strcmp(name, "pane-border-indicators") == 0) {
+		RB_FOREACH(w, windows, &windows) {
+			TAILQ_FOREACH(wp, &w->panes, entry) {
+				u_int	screen_sx, screen_sy;
+
+				/*
+				 * Resize screen buffer for box mode. The PTY
+				 * size is handled by window_pane_send_resize.
+				 */
+				if (window_pane_box_mode(wp) &&
+				    wp->sx >= 3 && wp->sy >= 3) {
+					screen_sx = wp->sx - 2;
+					screen_sy = wp->sy - 2;
+				} else {
+					screen_sx = wp->sx;
+					screen_sy = wp->sy;
+				}
+				screen_resize(&wp->base, screen_sx, screen_sy,
+				    wp->base.saved_grid == NULL);
+				window_pane_send_resize(wp, wp->sx, wp->sy);
+			}
+		}
+	}
 	if (strcmp(name, "codepoint-widths") == 0)
 		utf8_update_width_cache();
 	if (strcmp(name, "input-buffer-size") == 0)