Fix mobile Safari reload loop by persisting config cache in window object

- Store config cache in window.__configCache instead of module-level variable to maintain persistence across Vue reactivity cycles
- Fix cardClasses ref to store computed value instead of function
- Add client plugin for config cache initialization
- Add documentation for mobile Safari reload loop fix

pages/signup.vue

@@ -234,13 +234,13 @@ useHead({
 
 // Static CSS classes - computed once, never reactive
 const containerClasses = ref(getDeviceCssClasses('signup-container'));
-const cardClasses = ref(() => {
+const cardClasses = ref((() => {
   const classes = ['signup-card'];
   if (deviceInfo.isMobileSafari) {
     classes.push('performance-optimized', 'no-backdrop-filter');
   }
   return classes.join(' ');
-});
+})()); // Execute immediately and store the result, not the function
 
 // Form data - individual refs to prevent Vue reactivity corruption
 const firstName = ref('');