Fix red main: give inspect.silhouette an icon; scroll to Colour vision in its scenario (#2134)

Preferences > Graphics grew a row (Grid follows the view, #2035), so the Colour vision
control now sits below the fold of the fixed-size page; the scenario scrolls to it.
The page itself already scrolls for real users.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/tests/scenarios/colour_vision_setting.json

@@ -3,6 +3,7 @@
   {"click": {"handle": "titlebar:preferences"}},
   {"expect": {"prefs": {"dialog": "General", "color_vision": "off"}, "same_width": "prefs:window"}},
   {"widget": "Graphics"},
+  {"call": "ui.scroll", "params": {"x": 800, "y": 500, "delta": -3000}},
   {"expect": {"shown": "Colour vision", "prefs": {"dialog": "Graphics"}, "same_width": "prefs:window"}},
   {"widget": "Off"},
   {"widget": "Deutan"},
@@ -11,12 +12,14 @@
   {"expect": {"prefs": {"dialog": null, "color_vision": "off"}}},
   {"key": "Comma", "cmd": true},
   {"widget": "Graphics"},
+  {"call": "ui.scroll", "params": {"x": 800, "y": 500, "delta": -3000}},
   {"widget": "Off"},
   {"widget": "Tritan"},
   {"widget": "OK"},
   {"expect": {"prefs": {"dialog": null, "color_vision": "tritan"}}},
   {"key": "Comma", "cmd": true},
   {"widget": "Graphics"},
+  {"call": "ui.scroll", "params": {"x": 800, "y": 500, "delta": -3000}},
   {"expect": {"shown": "Tritan", "same_width": "prefs:window"}},
   {"widget": "Restore Defaults"},
   {"expect": {"prefs": {"color_vision": "off"}}},