Give the tests a visible inactive border colour

The integration test config asks for `black` inactive borders. In a
recording that comes out as a mid grey, because the recording theme
remaps the terminal's black to #7a7a7a. In an ordinary terminal it is
real black, so when you watch a test with `just e2e-cli` the inactive
frames all but disappear against the background.

Name the grey directly instead (but a little bit brighter than the #7a
we had before), so that the frames look the same either way.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

test/default_test_config/config.yml

@@ -8,8 +8,11 @@ gui:
     activeBorderColor:
     - green
     - bold
+    # Not the "black" of the terminal palette: that is a real black in an
+    # ordinary terminal, which makes the frames all but invisible when you run
+    # a test with `just e2e-cli`.
     inactiveBorderColor:
-    - black
+    - '#999999'
   # Not important in tests but it creates clutter in demos
   showRandomTip: false
   animateExplosion: false # takes too long