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).

tools/theme-shot.nix

@@ -17,7 +17,7 @@ let
     overlays = [ flake.overlays.default ];
     config.allowUnfree = true;
   };
-  slug = let s = builtins.getEnv "THEME"; in if s == "" then "tokyo-night" else s;
+  slug = let s = builtins.getEnv "THEME"; in if s == "" then "boreal" else s;
 in
 pkgs.testers.runNixOSTest {
   name = "theme-shot-${slug}";