fix(theme): destructive buttons were invisible after #98 (1.03:1)

e9dd3d1 pinned dialog button labels to base00 for dark palettes. That is
correct for suggested-action, whose background adw-gtk3 builds opaquely as
mix(white,@accent_bg_color,0.9) — but destructive-action is the one class
whose background is derived from currentColor:

  background-color: mix(@destructive_color, alpha(currentColor, 0.15), 0.9)

so pinning the label dragged the background down with it. On Boreal the
"Delete" button of a GtkMessageDialog rendered #21272F on #252931 —
1.03:1, invisible — and light palettes hit the same 1.03:1. The most
safety-critical button in a confirmation dialog was the one that vanished.

Pin the destructive background to the same opaque pastel ladder
suggested-action uses. Enabled-only (:not(.flat):not(:disabled)): the
sheet's disabled chrome already renders legibly and must keep it.

Boreal 1.03 -> 6.19:1, summer-day 1.03 -> 2.74:1 (= upstream's
white-on-accent convention; the residual light-mode weakness is #130).

V2: GtkMessageDialog rendered under the real HM session and viewed —
boreal before/after, summer-night (whole-swap), summer-day (light).
Scripted checks stayed green throughout and never saw this; only the
render did. Follow-ups #129 (permanent guard) and #130 filed.

docs/ROADMAP.md

@@ -432,6 +432,16 @@ Design/decision records and a running log of shipped work (items marked
   sheet hardcodes near-black button text → black-on-dark in GTK dialogs
   (Boreal). Fix: force `adw-gtk3-dark` / `adw-gtk3` from palette mode,
   prefer-dark, and pin dialog button colours in Stylix extraCss.
+  **Follow-up (same day):** pinning the label colour silently broke
+  `destructive-action` — adw-gtk3 derives only *that* background from
+  `currentColor` (`mix(@destructive_color, alpha(currentColor,…))`, where
+  suggested-action uses an opaque `mix(white,@accent_bg,…)`), so the dark
+  label dragged the background dark with it: "Delete" at 1.03:1, invisible,
+  in both polarities. Destructive backgrounds are now pinned to the same
+  opaque pastel ladder (enabled-only). V2: dialog rendered under boreal,
+  summer-night and summer-day. Lesson: a config-level assertion
+  ("settings.ini says adw-gtk3-dark") cannot see a colour bug — only a
+  render can.
 - ✓ **Suspend-then-hibernate (#115):** human decision — **1h** delay, **battery
   only**, Preferences enable/disable. Mechanism: `suspend-then-hibernate` +
   `HibernateDelaySec=1h` + `HibernateOnACPower=false`; lid