fix: backend-clear called with framebuffer instead of backend

Main loop was calling (backend-clear curr-fb) where curr-fb is a
framebuffer array. Changed to (backend-clear be) using the cl-tty
backend, which writes the terminal clear escape sequence.

org/channel-tui-main.org

@@ -885,7 +885,7 @@ Event handlers + daemon I/O + main loop.
                          (add-msg :system "Search exited")))
                      (t (on-key ch)))))))
             (when (or (first (st :dirty)) (second (st :dirty)) (third (st :dirty)))
-              (cl-tty.backend:backend-clear curr-fb)
+              (cl-tty.backend:backend-clear be)
               (redraw curr-fb w h)
               (cl-tty.rendering:flush-framebuffer prev-fb curr-fb be)
               (rotatef prev-fb curr-fb))