Preview: Block ausserhalb des Formulars, Validierung und Feinschliff

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/lib/previewEnv.js

@@ -22,3 +22,8 @@ export function isDevPreview(p) {
   if (p.mode === 'dev') return true
   return (p.mode || 'auto') === 'auto' && typeof p.detected === 'string' && p.detected.startsWith('dev:')
 }
+
+export const MAX_ENV_VARS = 30
+export function envValueTooLong(value) {
+  return new TextEncoder().encode(String(value ?? '')).length > 4096
+}