fix(theme): #66 fail-closed theme-state errors at mkFlake/read

Missing, empty, or non-object theme-state.json now throws a short
message pointing at the template and `nomarchy-theme-sync validate`
instead of a raw readFile/fromJSON stack. Shared reader:
modules/theme-state-read.nix — wired in theme.nix, plymouth/timezone/
hardware, and lib.mkFlake (seq-forced early gate). Field-level schema
in theme.nix kept; unknown border roles still fail there; resolveColor
no longer falls through on missing palette keys. Defaults added for
settings.displayProfile / displayProfileAuto.

Verified: V0 nix flake check --no-build; negative evals (missing,
empty, [], bad border role); good-path template mode + displayProfile
defaults.

modules/nixos/plymouth.nix

@@ -12,7 +12,7 @@ let
 
   state =
     if cfg.stateFile != null
-    then builtins.fromJSON (builtins.readFile cfg.stateFile)
+    then import ../theme-state-read.nix { inherit lib; } cfg.stateFile
     else { };
   colorOf = key: fallback: lib.removePrefix "#" ((state.colors or { }).${key} or fallback);
   # Fallbacks match the distro default theme (Boreal) when stateFile is null.