feat(ui): draw pane frames, page keys and a scroll thumb

tests/test_theme.py

@@ -52,8 +52,13 @@ async def test_backgrounds_use_terminal_default_and_borders_are_ansi() -> None:
             assert widget.styles.background.ansi == -1, widget_type
             assert widget.styles.color.ansi == -1, widget_type
 
-        border = app.query_one(StationList).styles.border.top
-        assert border[1].ansi is not None  # an ANSI palette index, not RGB
+        # panes draw their own frames now: dim edges, gradient corners
+        pane = app.query_one(StationList)
+        assert pane.styles.border.top[0] == ""  # no framework border any more
+        framed = pane.render().renderables
+        corners = [str(span.style) for span in framed[0].spans]
+        assert framed[0].plain.startswith("╭")
+        assert any(style.strip() for style in corners)
 
         await pilot.press("slash")
         await pilot.pause()