fix(choose-fonts): guard preview rendering against partial terminal color query

The choose-fonts kitten queries the terminal for the user's foreground and
background colors at startup by sending a DCS request. The terminal responds
with separate DCS response strings — one per field — which arrive
asynchronously through on_query_response.

The background handler unconditionally called draw_screen() after storing its
value. If the terminal's DCS response for "background" arrived before the
response for "foreground", draw_screen() fired with text_style.Foreground
still at its Go zero value ("").

The font list preview (list.go) only guarded against empty Background, not
empty Foreground, so it passed the check and sent a render_family_samples
command to the Python backend with foreground="". The backend's to_color('')
raised ValueError: Invalid color name: '', crashing the kitten.

The faces pane (faces.go) launched a render_family_samples goroutine
unconditionally when its preview cache was empty, with no guard at all.

Fix
---
1. ui.go — Only trigger draw_screen() from the foreground or background
   handler when the counterpart field is already populated, so the UI
   never renders with a partial TextStyle.

2. list.go — Require both Foreground and Background to be non-empty
   before rendering the font preview, not just Background alone.

3. faces.go — Skip the render_family_samples goroutine if either color
   field is still empty. The preview cache stays empty, so the next
   redraw (triggered when the counterpart query response arrives, or
   by on_wakeup) retries with full colors.

kittens/choose_fonts/faces.go

@@ -52,6 +52,9 @@ func (self *faces) draw_screen() (err error) {
 	previews, found := self.preview_cache[key]
 	if !found {
 		self.preview_cache[key] = make(map[string]RenderedSampleTransmit)
+		if self.handler.text_style.Foreground == "" || self.handler.text_style.Background == "" {
+			return
+		}
 		go func() {
 			var r map[string]RenderedSampleTransmit
 			s := key.settings

kittens/choose_fonts/list.go

@@ -122,7 +122,7 @@ func (self *FontList) draw_family_summary(start_x int, sz loop.ScreenSize) (err
 		lp.QueueWriteString(line)
 		y++
 	}
-	if self.handler.text_style.Background != "" {
+	if self.handler.text_style.Foreground != "" && self.handler.text_style.Background != "" {
 		return self.draw_preview(start_x, y, sz)
 	}
 	return

kittens/choose_fonts/ui.go

@@ -139,9 +139,14 @@ func (h *handler) on_query_response(key, val string, valid bool) error {
 		}
 	case "foreground":
 		h.text_style.Foreground = val
+		if h.text_style.Background != "" {
+			return h.draw_screen()
+		}
 	case "background":
 		h.text_style.Background = val
-		return h.draw_screen()
+		if h.text_style.Foreground != "" {
+			return h.draw_screen()
+		}
 	}
 	return nil
 }