fix(core): handle null omnixy.preset everywhere it's interpolated

Same null-vs-missing bug as the fastfetch fix, across lib.nix (env var +
status echo), menus.nix, scripts.nix, and fastfetch's about screen:
`cfg.preset or "…"` returns null (not the fallback) because preset
defaults to null. Any consumer that doesn't set a preset hit 'cannot
coerce null to a string'. Replace with explicit null checks.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/scripts.nix

@@ -25,7 +25,7 @@ in
         echo "│  🐧 OS: NixOS $(nixos-version)"
         echo "│  🎨 Theme: ${cfg.theme}"
         echo "│  👤 User: ${cfg.user}"
-        echo "│  🏠 Preset: ${cfg.preset or "custom"}"
+        echo "│  🏠 Preset: ${if cfg.preset == null then "custom" else cfg.preset}"
         echo "│"
         echo "│  🔧 Uptime: $(uptime -p)"
         echo "│  💾 Memory: $(free -h | awk 'NR==2{printf "%.1f/%.1fGB (%.0f%%)", $3/1024/1024, $2/1024/1024, $3*100/$2}')"