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.
       '';
     };