feat(theme): make Boreal the distro default

Seed theme-state.json and the downstream template from themes/boreal.json
(including border + empty settings). Point eval/schema fallbacks at Boreal
(theme.nix preset, plymouth and live-ISO splash colors, theme-shot, sync
error examples). Docs/TESTING and RECOVERY follow the new default.

Verified: state matches boreal palette; theme-contrast all pass.
preview.png for boreal still missing (picker plain-name; V3 capture).

modules/nixos/options.nix

@@ -33,7 +33,7 @@
       description = ''
         theme-state.json for the system-side consumers (currently the
         Plymouth splash background). lib.mkFlake wires it automatically
-        from your flake; null falls back to the Tokyo Night base color.
+        from your flake; null falls back to the Boreal base color.
       '';
     };
 

modules/nixos/plymouth.nix

@@ -15,14 +15,15 @@ let
     then builtins.fromJSON (builtins.readFile cfg.stateFile)
     else { };
   colorOf = key: fallback: lib.removePrefix "#" ((state.colors or { }).${key} or fallback);
-  base = colorOf "base" "#1a1b26";
+  # Fallbacks match the distro default theme (Boreal) when stateFile is null.
+  base = colorOf "base" "#21272F";
   # Splash elements are recolored from the palette so they read on any
   # base, light or dark: foreground glyphs → text, field/track boxes →
   # surface (raised from base in both polarities), the progress fill →
   # accent. The shipped art is a fixed navy that vanished on dark themes.
-  text = colorOf "text" "#a9b1d6";
-  surface = colorOf "surface" "#32344a";
-  accent = colorOf "accent" "#7aa2f7";
+  text = colorOf "text" "#D3DAE0";
+  surface = colorOf "surface" "#303A46";
+  accent = colorOf "accent" "#B79BE8";
   magick = lib.getExe' pkgs.imagemagick "magick";
 
   # Plymouth's Window.SetBackgroundTopColor takes three floats in