feat: add platform assignment to user and vendor creation forms

User create page:
- When role=admin, show super admin toggle
- If not super admin, show platform multi-select
- Admin users created via /api/v1/admin/admin-users endpoint
- Vendor users created via existing /admin/users endpoint

Vendor create page:
- Add platform selection section
- Vendors can be assigned to multiple platforms on creation
- Update VendorCreate schema to accept platform_ids
- Update AdminService.create_vendor() to create VendorPlatform records

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

static/admin/js/vendor-create.js

@@ -23,6 +23,9 @@ function adminVendorCreate() {
         companies: [],
         loadingCompanies: true,
 
+        // Platforms list for selection
+        platforms: [],
+
         // Form data matching VendorCreate schema
         formData: {
             company_id: '',
@@ -32,7 +35,8 @@ function adminVendorCreate() {
             description: '',
             letzshop_csv_url_fr: '',
             letzshop_csv_url_en: '',
-            letzshop_csv_url_de: ''
+            letzshop_csv_url_de: '',
+            platform_ids: []
         },
 
         // UI state
@@ -49,7 +53,10 @@ function adminVendorCreate() {
 
             try {
                 vendorCreateLog.info('Initializing vendor create page');
-                await this.loadCompanies();
+                await Promise.all([
+                    this.loadCompanies(),
+                    this.loadPlatforms()
+                ]);
             } catch (error) {
                 vendorCreateLog.error('Failed to initialize vendor create:', error);
             }
@@ -70,6 +77,18 @@ function adminVendorCreate() {
             }
         },
 
+        // Load platforms for selection
+        async loadPlatforms() {
+            try {
+                const response = await apiClient.get('/admin/platforms');
+                this.platforms = response.platforms || response.items || [];
+                vendorCreateLog.debug('Loaded platforms:', this.platforms.length);
+            } catch (error) {
+                vendorCreateLog.error('Failed to load platforms:', error);
+                this.platforms = [];
+            }
+        },
+
         // Auto-generate subdomain from vendor name
         autoGenerateSubdomain() {
             if (!this.formData.name) {
@@ -124,6 +143,11 @@ function adminVendorCreate() {
                     payload.letzshop_csv_url_de = this.formData.letzshop_csv_url_de;
                 }
 
+                // Add platform assignments
+                if (this.formData.platform_ids && this.formData.platform_ids.length > 0) {
+                    payload.platform_ids = this.formData.platform_ids.map(id => parseInt(id));
+                }
+
                 const response = await apiClient.post('/admin/vendors', payload);
 
                 vendorCreateLog.info('Vendor created successfully:', response.vendor_code);
@@ -147,7 +171,8 @@ function adminVendorCreate() {
                     description: '',
                     letzshop_csv_url_fr: '',
                     letzshop_csv_url_en: '',
-                    letzshop_csv_url_de: ''
+                    letzshop_csv_url_de: '',
+                    platform_ids: []
                 };
 
                 // Scroll to top to show success message