fix(ui): gate redraw on suspend flag to stop bleed-through over editor

The editor rewrite (async g_child_watch) dropped the background_mode
guard that paused the main loop while an external editor (e.g. nano)
runs. ui_update/ui_resize/ui_redraw instead self-guarded on isendwin(),
which is unreliable: any doupdate() during the editor session (e.g. the
readline redisplay after the alt+c keypress) clears it, after which the
~10Hz main-loop ui_update() repaints the status bar, chat window and
input pad over the editor. The most visible artifact is the per-minute
status-bar clock digit bleeding through onto the editor screen.

Track suspension explicitly with a ui_suspended flag set in
ui_suspend()/ui_resume() and gate ui_update/ui_resize/ui_redraw on it
(kept alongside isendwin() for the shutdown/closed-screen case). The
flag stays TRUE for the whole editor session regardless of isendwin
flapping, so nothing repaints over the editor.

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 +64,14 @@ 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).
+static gboolean
+_ui_redraw_suspended(void)
+{
+    return ui_suspended || isendwin();
+}
+
 static void
 _ui_close(void)
 {
@@ -120,8 +129,7 @@ ui_sigwinch_handler(int sig)
 void
 ui_update(void)
 {
-    // UI is suspended
-    if (isendwin()) {
+    if (_ui_redraw_suspended()) {
         return;
     }
 
@@ -179,6 +187,7 @@ ui_reset_idle_time(void)
 void
 ui_suspend(void)
 {
+    ui_suspended = TRUE;
     inp_suspend();
     endwin();
 }
@@ -186,6 +195,7 @@ ui_suspend(void)
 void
 ui_resume(void)
 {
+    ui_suspended = FALSE;
     refresh();
     inp_resume();
 }
@@ -193,8 +203,7 @@ ui_resume(void)
 void
 ui_resize(void)
 {
-    // UI is suspended
-    if (isendwin()) {
+    if (_ui_redraw_suspended()) {
         return;
     }
 
@@ -216,8 +225,7 @@ ui_resize(void)
 void
 ui_redraw(void)
 {
-    // UI is suspended
-    if (isendwin()) {
+    if (_ui_redraw_suspended()) {
         return;
     }