fix(ui): measure dead pad space from cursor instead of absolute height

Remove the /autoping warning subcommand and PREF_AUTOPING_WARNING
preference entirely. The warning was shown at connect time when
autoping was disabled but the server supported XEP-0199 ping.

The pad threshold logic previously used an absolute height
(PAD_THRESHOLD=12000) which was above the buffer cap and never
fired during normal scrolling. Replace with a dead-space measurement
(cursor position minus live buffer lines) that only triggers a redraw
when dead space exceeds PAD_DEAD_SPACE_LIMIT (2000 lines).

Author: jabber.developer2 <jabber.developer2@jabber.space>

src/ui/window.c

@@ -43,7 +43,7 @@
 #include "ai/ai_client.h"
 
 static const int PAD_MIN_HEIGHT = 100;
-static const int PAD_THRESHOLD = 12000; // above buffer cap (~9000): reclaims dead pad space, never fires while scrolling
+static const int PAD_DEAD_SPACE_LIMIT = 2000; // reclaim once the pad holds this much dead space (cursor past live buffer); size-independent, so it never fires while scrolling
 static gboolean _in_redraw = FALSE;
 
 static void
@@ -56,9 +56,10 @@ _win_ensure_pad_capacity(ProfWin* window, WINDOW* win, int lines_needed)
     int cur_height = getmaxy(win);
     int cur_width = getmaxx(win);
     if (lines_needed >= cur_height - 1) {
-        // If we are getting too large, trigger a redraw to clean up old lines
-        // but only if we are not already in a redraw process.
-        if (window && cur_height >= PAD_THRESHOLD && !_in_redraw) {
+        // redraw to reclaim dead pad space (cursor far past live buffer, e.g. a long
+        // append-only session); dead space never accrues while scrolling, only here
+        int dead = window ? getcury(win) - window->layout->buffer->lines : 0;
+        if (window && dead > PAD_DEAD_SPACE_LIMIT && !_in_redraw) {
             win_redraw(window);
         } else {
             // resize to required lines + some buffer for next messages