fix(theme): dark adw-gtk3 for dark palettes — GTK dialog buttons (#98)

Stylix always used the light adw-gtk3 sheet with recolored tokens; that
sheet hardcodes near-black button labels, which became black-on-dark in
Boreal GTK dialogs. Force adw-gtk3-dark (and prefer-dark) for dark mode,
and pin dialog button label colours in Stylix extraCss.

docs/ROADMAP.md

@@ -427,6 +427,11 @@ Design/decision records and a running log of shipped work (items marked
   default `$TERMINAL` / SUPER+Return, doctor + calendar + what-changed use
   `kitty --class=com.nomarchy.*`. Ghostty module and install-time GL probe
   removed. V3: Acer SUPER+Return + doctor float.
+- ✓ **GTK dialog button labels (#98):** Stylix always set `gtk-theme-name=
+  adw-gtk3` (light sheet) while recoloring tokens for dark palettes — light
+  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.
 - ✓ **Generation prune (#128):** weekly `nomarchy-gen-prune` (system + HM):
   delete only gens older than **14 days** that are also beyond the **3 most
   recent past** gens (current always kept). Stock `nix.gc` no longer uses