fix(ui): reclaim pad by dead space instead of absolute height

Variant A (PAD_THRESHOLD=12000) bounds the pad but still re-enters win_redraw() from the print path on every message once the *live* buffer renders >= the threshold — reachable with a single oversized message (huge paste, narrow terminal, crafted incoming), reintroducing the per-message redraw storm.

Trigger the reclaim redraw on dead space (getcury - buffer->lines) instead of absolute pad height. After a redraw the cursor equals buffer->lines, so dead space is 0 regardless of buffer size: the reclaim can never fire while scrolling, and an oversized message no longer storms (and is shown in full rather than clipped). Dead space only accrues in a long append-only session that trimmed old entries — exactly when reclaiming is wanted.

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 ? lines_needed - 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