fix(ui): raise pad redraw threshold above buffer cap to fix history scroll

PAD_THRESHOLD (3000) sat below the buffer line cap (PAD_SIZE - PAD_SIZE/10 = 9000), so _win_ensure_pad_capacity() fired a full win_redraw() from the hot print path on every message once a chat exceeded 3000 rendered lines. During history paging this turned one redraw per fetch into dozens, corrupting per-entry y_start_pos (offset jumps, dropped/skipped messages) and causing ~10s lag per page. The flat-file backend was byte-identical across the regression; the fault was purely in the UI pad path from the upstream merge.

Raise PAD_THRESHOLD to 12000 (above the cap) so the reclaim redraw only fires to drop dead pad space in long append-only sessions, never during scrolling. Also size the pad once to buffer->lines+100 in win_redraw() instead of shrinking to PAD_MIN_HEIGHT and regrowing per entry, removing the residual per-redraw wresize churn.

src/ui/window.c

@@ -2175,7 +2175,7 @@ win_redraw(ProfWin* window)
     // size the pad to fit the whole buffer (plus headroom) and erase it
     int cols = getmaxx(window->layout->win);
     int needed = window->layout->buffer->lines + 100;
-    wresize(window->layout->win, needed > PAD_MIN_HEIGHT ? needed : PAD_MIN_HEIGHT, cols);
+    wresize(window->layout->win, MAX(needed, PAD_MIN_HEIGHT), cols);
     werase(window->layout->win);
 
     for (unsigned int i = 0; i < size; i++) {