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/init-alpine.js

@@ -257,4 +257,64 @@ function headerMessages() {
 }
 
 // Export to window
-window.headerMessages = headerMessages;
+window.headerMessages = headerMessages;
+
+/**
+ * Platform Settings Utility
+ * Provides cached access to platform-wide settings
+ */
+const PlatformSettings = {
+    // Cache key and TTL
+    CACHE_KEY: 'platform_settings_cache',
+    CACHE_TTL: 5 * 60 * 1000, // 5 minutes
+
+    /**
+     * Get cached settings or fetch from API
+     */
+    async get() {
+        try {
+            const cached = localStorage.getItem(this.CACHE_KEY);
+            if (cached) {
+                const { data, timestamp } = JSON.parse(cached);
+                if (Date.now() - timestamp < this.CACHE_TTL) {
+                    return data;
+                }
+            }
+
+            // Fetch from API
+            const response = await apiClient.get('/admin/settings/display/public');
+            const settings = {
+                rows_per_page: response.rows_per_page || 20
+            };
+
+            // Cache the result
+            localStorage.setItem(this.CACHE_KEY, JSON.stringify({
+                data: settings,
+                timestamp: Date.now()
+            }));
+
+            return settings;
+        } catch (error) {
+            console.warn('Failed to load platform settings, using defaults:', error);
+            return { rows_per_page: 20 };
+        }
+    },
+
+    /**
+     * Get rows per page setting
+     */
+    async getRowsPerPage() {
+        const settings = await this.get();
+        return settings.rows_per_page;
+    },
+
+    /**
+     * Clear the cache (call after saving settings)
+     */
+    clearCache() {
+        localStorage.removeItem(this.CACHE_KEY);
+    }
+};
+
+// Export to window
+window.PlatformSettings = PlatformSettings;