fix(ui): add authoritative suspend flag gating redraws over the editor

Builds on the upstream "guard terminal writes while suspended" fix. That
fix keeps isendwin() a valid "suspended" signal by guarding stray writes
(_inp_write, beep, flash) so the existing isendwin() checks in
ui_update/ui_resize/ui_redraw hold. That is correct but fail-open: any
future unguarded doupdate()/refresh() reachable during an editor session
clears isendwin() and the ~10Hz ui_update() repaints over the editor
again (the original bug class).

Track suspension explicitly with a ui_suspended flag set in
ui_suspend()/ui_resume() and gate the three redraw entry points on
ui_suspended || isendwin() via _ui_redraw_suspended(). The flag is set
exactly at the suspend/resume boundary, so the dominant repaint path
fails safe regardless of isendwin() flapping. isendwin() is kept in the
condition for the shutdown/closed-screen case.

src/ui/core.c

@@ -54,6 +54,7 @@
 
 static int inp_size;
 static gboolean perform_resize = FALSE;
+static gboolean ui_suspended = FALSE;
 static GTimer* ui_idle_time;
 static WINDOW* main_scr;
 
@@ -63,6 +64,16 @@ static Display* display;
 
 static void _ui_draw_term_title(void);
 
+// Skip drawing while an external editor is active (ui_suspended) or after the
+// screen has been closed (endwin). The flag is authoritative and does not rely
+// on isendwin() staying TRUE, which a stray doupdate() during the session could
+// otherwise clear.
+static gboolean
+_ui_redraw_suspended(void)
+{
+    return ui_suspended || isendwin();
+}
+
 static void
 _ui_close(void)
 {
@@ -120,8 +131,7 @@ ui_sigwinch_handler(int sig)
 void
 ui_update(void)
 {
-    // UI is suspended
-    if (isendwin()) {
+    if (_ui_redraw_suspended()) {
         return;
     }
 
@@ -179,6 +189,7 @@ ui_reset_idle_time(void)
 void
 ui_suspend(void)
 {
+    ui_suspended = TRUE;
     inp_suspend();
     doupdate();
     endwin();
@@ -187,6 +198,7 @@ ui_suspend(void)
 void
 ui_resume(void)
 {
+    ui_suspended = FALSE;
     refresh();
     inp_resume();
 }
@@ -210,8 +222,7 @@ ui_flash(void)
 void
 ui_resize(void)
 {
-    // UI is suspended
-    if (isendwin()) {
+    if (_ui_redraw_suspended()) {
         return;
     }
 
@@ -233,8 +244,7 @@ ui_resize(void)
 void
 ui_redraw(void)
 {
-    // UI is suspended
-    if (isendwin()) {
+    if (_ui_redraw_suspended()) {
         return;
     }