Give the right-click menu the darkest surface and Oxide's selection

The menu now sits on the darkest surface (the frame color) behind a faint
border, and its highlighted item takes Oxide's selected style: a dark green
fill (their surface-accent) with green text. Mouse and keyboard share that
one highlight, so a hovered row and a keyboard-selected row can no longer
light up at the same time.

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

docs/architecture.md

@@ -317,6 +317,11 @@ headers `--toolbar-3` (`color-mix` steps declared once in `:root`, so every
 theme follows) — then `--background` (file lists, editor text, forms;
 `--row-alt` stripes odd file rows, skipped on the cursor/selected row so two
 `bg-*` utilities never fight) and `--card` (popovers, dialogs, toasts). The
+right-click menu is the exception: the darkest surface (`bg-chrome`) behind a
+faint `border-border/60`, and its highlighted item takes Oxide's selected style —
+`--primary-surface` fill (Oxide surface-accent: green-200 dark, green-1200
+light) with `text-primary`; mouse and keyboard share that one highlight
+(`activeIndex`). The
 ACTIVE panel is marked by a 0.125rem `--primary` line along its top edge (an
 inset box-shadow on the session strip, so switching panels never shifts
 layout). Dark: frame 0.162 < list 0.195 < header 0.275 → 0.248 → 0.221 < card