fix(tax): All 4 issues resolved

## Fixes:

1. ✅ Suggested rates now inside Tax Rates card as help notice
   - Shows as blue notice box
   - Only shows rates not yet added
   - Auto-hides when all suggested rates added

2. ✅ Add Rate button now works
   - Fixed mutation to properly invalidate queries
   - Shows success toast
   - Updates list immediately

3. ✅ Add Tax Rate dialog no longer blank
   - Replaced shadcn Select with native select
   - Form now submits properly
   - All fields visible

4. ✅ Tax toggle now functioning
   - Changed onChange to onCheckedChange
   - Added required id prop
   - Properly typed checked parameter

## Additional:
- Added api.put() method to api.ts
- Improved UX with suggested rates as contextual help

admin-spa/src/lib/api.ts

@@ -59,6 +59,14 @@ export const api = {
     });
   },
 
+  async put(path: string, body?: any) {
+    return api.wpFetch(path, {
+      method: 'PUT',
+      headers: { 'Content-Type': 'application/json' },
+      body: body != null ? JSON.stringify(body) : undefined,
+    });
+  },
+
   async del(path: string) {
     return api.wpFetch(path, { method: 'DELETE' });
   },