fix: add nil guards on w and h in tui-main before initial render

backend-size can return nil for height (especially when the /dev/tty
ioctl fallback hasn't been compiled in yet). view functions had nil
guards but the direct (- h 4) calls in tui-main's initial render
crashed before reaching them.

org/channel-tui-main.org

@@ -918,6 +918,9 @@ Event handlers + daemon I/O + main loop.
                                               :stderr nil))
                (tty-in (uiop:process-info-output cat-proc)))
           (add-msg :system (format nil "* cat pid=~a *" (uiop:process-info-pid cat-proc)))
+          ;; Guard against nil w/h from backend-size
+          (setq w (or (and (numberp w) (> w 0) w) 80)
+                h (or (and (numberp h) (> h 0) h) 24))
           ;; Log backend info and terminal dimensions
           (cl-tty.backend:backend-clear be)
           (view-status be w h)