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/shared/js/vendor-selector.js

@@ -66,7 +66,7 @@ function waitForTomSelect(callback, maxRetries = 20, retryDelay = 100) {
  * @param {number} options.minChars - Minimum characters before search (default: 2)
  * @param {number} options.maxOptions - Maximum options to show (default: 50)
  * @param {string} options.placeholder - Placeholder text
- * @param {string} options.apiEndpoint - API endpoint for search (default: '/api/v1/admin/vendors')
+ * @param {string} options.apiEndpoint - API endpoint for search (default: '/admin/vendors')
  * @returns {Object} Controller object with setValue() and clear() methods
  */
 function initVendorSelector(selectElement, options = {}) {
@@ -79,7 +79,7 @@ function initVendorSelector(selectElement, options = {}) {
         minChars: options.minChars || 2,
         maxOptions: options.maxOptions || 50,
         placeholder: options.placeholder || selectElement.getAttribute('placeholder') || 'Search vendor by name or code...',
-        apiEndpoint: options.apiEndpoint || '/api/v1/admin/vendors',
+        apiEndpoint: options.apiEndpoint || '/admin/vendors',  // Note: apiClient adds /api/v1 prefix
         onSelect: options.onSelect || (() => {}),
         onClear: options.onClear || (() => {})
     };