fix: Critical data structure and mutation bugs

## 🐛 Critical Fixes

### Issue 1: Toggling One Channel Affects Both
**Problem:** Disabling email disabled both email and push
**Root Cause:** Optimistic update with `onSettled` refetch caused race condition
**Fix:** Removed optimistic update, use server response directly

**Before:**
```ts
onMutate: async () => {
  // Optimistic update
  queryClient.setQueryData(...)
}
onSettled: () => {
  // This refetch caused race condition
  queryClient.invalidateQueries(...)
}
```

**After:**
```ts
onSuccess: (data, variables) => {
  // Update cache with verified server response
  queryClient.setQueryData([...], (old) =>
    old.map(channel =>
      channel.id === variables.channelId
        ? { ...channel, enabled: data.enabled }
        : channel
    )
  );
}
```

### Issue 2: Events Cannot Be Enabled
**Problem:** All event channels disabled and cannot be enabled
**Root Cause:** Wrong data structure in `update_event()`

**Before:**
```php
$settings[$event_id][$channel_id] = [...];
// Saved as: { "order_placed": { "email": {...} } }
```

**After:**
```php
$settings[$event_id]['channels'][$channel_id] = [...];
// Saves as: { "order_placed": { "channels": { "email": {...} } } }
```

### Issue 3: POST Data Not Parsed
**Problem:** Event updates not working
**Root Cause:** Using `get_param()` instead of `get_json_params()`
**Fix:** Changed to `get_json_params()` in `update_event()`

### What Was Fixed

1. ✅ Channel toggles work independently
2. ✅ No race conditions from optimistic updates
3. ✅ Event channel data structure matches get_events
4. ✅ Event toggles save correctly
5. ✅ POST data parsed properly
6. ✅ Boolean type enforcement

### Data Structure

**Correct Structure:**
```php
[
  'order_placed' => [
    'channels' => [
      'email' => ['enabled' => true, 'recipient' => 'admin'],
      'push' => ['enabled' => false, 'recipient' => 'admin']
    ]
  ]
]
```

---

**All toggles should now work correctly!** ✅

admin-spa/src/routes/Settings/Notifications/Channels.tsx

@@ -47,38 +47,25 @@ export default function NotificationChannels() {
   // Toggle channel mutation
   const toggleChannelMutation = useMutation({
     mutationFn: async ({ channelId, enabled }: { channelId: string; enabled: boolean }) => {
-      return api.post('/notifications/channels/toggle', { channelId, enabled });
+      const response = await api.post('/notifications/channels/toggle', { channelId, enabled });
+      return response;
     },
-    onMutate: async ({ channelId, enabled }) => {
-      // Cancel outgoing refetches
-      await queryClient.cancelQueries({ queryKey: ['notification-channels'] });
-      
-      // Snapshot previous value
-      const previousChannels = queryClient.getQueryData(['notification-channels']);
-      
-      // Optimistically update
+    onSuccess: (data, variables) => {
+      // Update cache with server response
       queryClient.setQueryData(['notification-channels'], (old: any) => {
         if (!old) return old;
         return old.map((channel: any) => 
-          channel.id === channelId ? { ...channel, enabled } : channel
+          channel.id === variables.channelId 
+            ? { ...channel, enabled: data.enabled } 
+            : channel
         );
       });
-      
-      return { previousChannels };
-    },
-    onSuccess: () => {
       toast.success(__('Channel updated'));
     },
-    onError: (error: any, variables, context: any) => {
-      // Rollback on error
-      if (context?.previousChannels) {
-        queryClient.setQueryData(['notification-channels'], context.previousChannels);
-      }
-      toast.error(error?.message || __('Failed to update channel'));
-    },
-    onSettled: () => {
-      // Refetch after mutation
+    onError: (error: any) => {
+      // Refetch on error to sync with server
       queryClient.invalidateQueries({ queryKey: ['notification-channels'] });
+      toast.error(error?.message || __('Failed to update channel'));
     },
   });