feat: add PlatformSettings for pagination and vendor filter improvements

Platform Settings:
- Add PlatformSettings utility in init-alpine.js with 5-min cache
- Add Display tab in /admin/settings for rows_per_page config
- Integrate PlatformSettings.getRowsPerPage() in all paginated pages
- Standardize default per_page to 20 across all admin pages
- Add documentation at docs/frontend/shared/platform-settings.md

Architecture Rules:
- Add JS-010: enforce PlatformSettings usage for pagination
- Add JS-011: enforce standard pagination structure
- Add JS-012: detect double /api/v1 prefix in apiClient calls
- Implement all rules in validate_architecture.py

Vendor Filter (Tom Select):
- Add vendor filter to marketplace-products, vendor-products,
  customers, inventory, and vendor-themes pages
- Add selectedVendor display panel with clear button
- Add localStorage persistence for vendor selection
- Fix double /api/v1 prefix in vendor-selector.js

Bug Fixes:
- Remove duplicate PlatformSettings from utils.js
- Fix customers.js pagination structure (page_size → per_page)
- Fix code-quality-violations.js pagination structure

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

static/admin/js/vendor-themes.js

@@ -24,6 +24,13 @@ function adminVendorThemes() {
         vendors: [],
         selectedVendorCode: '',
 
+        // Selected vendor for filter (Tom Select)
+        selectedVendor: null,
+        vendorSelector: null,
+
+        // Search/filter
+        searchQuery: '',
+
         async init() {
             // Guard against multiple initialization
             if (window._adminVendorThemesInitialized) {
@@ -31,13 +38,86 @@ function adminVendorThemes() {
             }
             window._adminVendorThemesInitialized = true;
 
-            // Call parent init first
-            const parentInit = data().init;
-            if (parentInit) {
-                await parentInit.call(this);
+            vendorThemesLog.info('Vendor Themes init() called');
+
+            // Initialize vendor selector (Tom Select)
+            this.$nextTick(() => {
+                this.initVendorSelector();
+            });
+
+            // Check localStorage for saved vendor
+            const savedVendorId = localStorage.getItem('vendor_themes_selected_vendor_id');
+            if (savedVendorId) {
+                vendorThemesLog.info('Restoring saved vendor:', savedVendorId);
+                await this.loadVendors();
+                // Restore vendor after vendors are loaded
+                setTimeout(async () => {
+                    await this.restoreSavedVendor(parseInt(savedVendorId));
+                }, 200);
+            } else {
+                await this.loadVendors();
             }
 
-            await this.loadVendors();
+            vendorThemesLog.info('Vendor Themes initialization complete');
+        },
+
+        /**
+         * Restore saved vendor from localStorage
+         */
+        async restoreSavedVendor(vendorId) {
+            try {
+                const vendor = await apiClient.get(`/admin/vendors/${vendorId}`);
+                if (this.vendorSelector && vendor) {
+                    // Use the vendor selector's setValue method
+                    this.vendorSelector.setValue(vendor.id, vendor);
+
+                    // Set the filter state
+                    this.selectedVendor = vendor;
+
+                    vendorThemesLog.info('Restored vendor:', vendor.name);
+                }
+            } catch (error) {
+                vendorThemesLog.warn('Failed to restore saved vendor, clearing localStorage:', error);
+                localStorage.removeItem('vendor_themes_selected_vendor_id');
+            }
+        },
+
+        /**
+         * Initialize vendor selector with Tom Select
+         */
+        initVendorSelector() {
+            if (!this.$refs.vendorSelect) {
+                vendorThemesLog.warn('Vendor select element not found');
+                return;
+            }
+
+            this.vendorSelector = initVendorSelector(this.$refs.vendorSelect, {
+                placeholder: 'Search vendor...',
+                onSelect: (vendor) => {
+                    vendorThemesLog.info('Vendor selected:', vendor);
+                    this.selectedVendor = vendor;
+                    // Save to localStorage
+                    localStorage.setItem('vendor_themes_selected_vendor_id', vendor.id.toString());
+                },
+                onClear: () => {
+                    vendorThemesLog.info('Vendor filter cleared');
+                    this.selectedVendor = null;
+                    // Clear from localStorage
+                    localStorage.removeItem('vendor_themes_selected_vendor_id');
+                }
+            });
+        },
+
+        /**
+         * Clear vendor filter
+         */
+        clearVendorFilter() {
+            if (this.vendorSelector) {
+                this.vendorSelector.clear();
+            }
+            this.selectedVendor = null;
+            // Clear from localStorage
+            localStorage.removeItem('vendor_themes_selected_vendor_id');
         },
 
         async loadVendors() {
@@ -56,6 +136,28 @@ function adminVendorThemes() {
             }
         },
 
+        /**
+         * Computed: Filtered vendors based on search and selected vendor
+         */
+        get filteredVendors() {
+            let filtered = this.vendors;
+
+            // If a vendor is selected via Tom Select, show only that vendor
+            if (this.selectedVendor) {
+                filtered = this.vendors.filter(v => v.id === this.selectedVendor.id);
+            }
+            // Otherwise filter by search query
+            else if (this.searchQuery) {
+                const query = this.searchQuery.toLowerCase();
+                filtered = this.vendors.filter(v =>
+                    v.name.toLowerCase().includes(query) ||
+                    (v.vendor_code && v.vendor_code.toLowerCase().includes(query))
+                );
+            }
+
+            return filtered;
+        },
+
         navigateToTheme() {
             if (!this.selectedVendorCode) {
                 return;