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/settings.js

@@ -17,7 +17,10 @@ function adminSettings() {
         saving: false,
         error: null,
         successMessage: null,
-        activeTab: 'logging',
+        activeTab: 'display',
+        displaySettings: {
+            rows_per_page: 20
+        },
         logSettings: {
             log_level: 'INFO',
             log_file_max_size_mb: 10,
@@ -41,6 +44,7 @@ function adminSettings() {
             try {
                 settingsLog.info('=== SETTINGS PAGE INITIALIZING ===');
                 await Promise.all([
+                    this.loadDisplaySettings(),
                     this.loadLogSettings(),
                     this.loadShippingSettings()
                 ]);
@@ -54,11 +58,52 @@ function adminSettings() {
             this.error = null;
             this.successMessage = null;
             await Promise.all([
+                this.loadDisplaySettings(),
                 this.loadLogSettings(),
                 this.loadShippingSettings()
             ]);
         },
 
+        async loadDisplaySettings() {
+            try {
+                const data = await apiClient.get('/admin/settings/display/rows-per-page');
+                this.displaySettings.rows_per_page = data.rows_per_page || 20;
+                settingsLog.info('Display settings loaded:', this.displaySettings);
+            } catch (error) {
+                settingsLog.error('Failed to load display settings:', error);
+                // Use default value on error
+                this.displaySettings.rows_per_page = 20;
+            }
+        },
+
+        async saveDisplaySettings() {
+            this.saving = true;
+            this.error = null;
+            this.successMessage = null;
+
+            try {
+                const data = await apiClient.put(`/admin/settings/display/rows-per-page?rows=${this.displaySettings.rows_per_page}`);
+                this.successMessage = data.message || 'Display settings saved successfully';
+
+                // Clear the cached platform settings so pages pick up the new value
+                if (window.PlatformSettings) {
+                    window.PlatformSettings.clearCache();
+                }
+
+                // Auto-hide success message after 5 seconds
+                setTimeout(() => {
+                    this.successMessage = null;
+                }, 5000);
+
+                settingsLog.info('Display settings saved successfully');
+            } catch (error) {
+                settingsLog.error('Failed to save display settings:', error);
+                this.error = error.response?.data?.detail || 'Failed to save display settings';
+            } finally {
+                this.saving = false;
+            }
+        },
+
         async loadLogSettings() {
             this.loading = true;
             this.error = null;