Mark the active panel with an accent line, not a tinted header

Tinting the whole header strip of the focused panel was too much: the
strip changed color every time focus moved. Both panels now share one
neutral header strip, and the active panel is marked by a thin accent
line along its top edge (an inset shadow, so switching panels never
shifts the layout).

Co-Authored-By: Claude <noreply@anthropic.com>

docs/gotchas.md

@@ -60,12 +60,13 @@ touching the adjacent code.
 
 - **A new surface token needs an AMOLED line, or AMOLED stops being
   all-black.** The two `[data-dark-tone='darker']` blocks re-declare every
-  surface (`--chrome`, `--toolbar`, `--toolbar-active`, `--row-alt`, …) as
-  `#000`; a token added only to the dark blocks shows through in AMOLED.
+  surface (`--chrome`, `--toolbar`, `--row-alt`, …) as `#000`; a token added
+  only to the dark blocks shows through in AMOLED.
 - **Never declare one token in both the AMOLED blocks and the accent dark
   formula blocks.** Both are (0,3,0), so source order decides and the later
-  accent block wins. That is why `--toolbar-active` lives in the base theme
-  blocks — its `var(--primary)` resolves per accent on `:root` anyway.
+  accent block wins. A token that must follow the accent belongs in the base
+  theme blocks instead — a `var(--primary)` there still resolves per accent on
+  `:root`.
 - **Hover/selected color is a translucent overlay (`--accent`).** An opaque
   hover vanishes on a surface of its own lightness — light's old 0.967 hover
   on the 0.965 header strip did exactly that. Keep `--accent` alpha-based when