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/users.js

@@ -28,7 +28,7 @@ function adminUsers() {
         },
         pagination: {
             page: 1,
-            per_page: 10,
+            per_page: 20,
             total: 0,
             pages: 0
         },
@@ -36,17 +36,22 @@ function adminUsers() {
         // Initialization
         async init() {
             usersLog.info('=== USERS PAGE INITIALIZING ===');
-            
+
             // Prevent multiple initializations
             if (window._usersInitialized) {
                 usersLog.warn('Users page already initialized, skipping...');
                 return;
             }
             window._usersInitialized = true;
-            
+
+            // Load platform settings for rows per page
+            if (window.PlatformSettings) {
+                this.pagination.per_page = await window.PlatformSettings.getRowsPerPage();
+            }
+
             await this.loadUsers();
             await this.loadStats();
-            
+
             usersLog.info('=== USERS PAGE INITIALIZATION COMPLETE ===');
         },