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>

.architecture-rules/frontend.yaml

@@ -145,6 +145,143 @@ javascript_rules:
       exceptions:
         - "utils.js"
 
+  - id: "JS-010"
+    name: "Use PlatformSettings for pagination rows per page"
+    severity: "error"
+    description: |
+      All pages with tables MUST use window.PlatformSettings.getRowsPerPage()
+      to load the platform-configured rows per page setting. This ensures
+      consistent pagination behavior across the entire admin and vendor interface.
+
+      The setting is configured at /admin/settings under the Display tab.
+      Settings are cached client-side for 5 minutes to minimize API calls.
+
+      Required pattern in init() method:
+        async init() {
+            // Guard against multiple initialization
+            if (window._pageNameInitialized) return;
+            window._pageNameInitialized = true;
+
+            // REQUIRED: Load platform settings for pagination
+            if (window.PlatformSettings) {
+                this.pagination.per_page = await window.PlatformSettings.getRowsPerPage();
+            }
+
+            await this.loadData();
+        }
+
+      WRONG (hardcoded pagination):
+        pagination: {
+            page: 1,
+            per_page: 50,  // Hardcoded!
+            total: 0
+        }
+
+      RIGHT (platform settings):
+        pagination: {
+            page: 1,
+            per_page: 20,  // Default, overridden by PlatformSettings
+            total: 0
+        }
+
+        async init() {
+            if (window.PlatformSettings) {
+                this.pagination.per_page = await window.PlatformSettings.getRowsPerPage();
+            }
+        }
+
+      Documentation: docs/frontend/shared/platform-settings.md
+    pattern:
+      file_pattern: "static/admin/js/**/*.js"
+      required_in_pages_with_pagination:
+        - "PlatformSettings\\.getRowsPerPage"
+        - "window\\.PlatformSettings"
+      exceptions:
+        - "init-alpine.js"
+        - "init-api-client.js"
+        - "settings.js"
+
+  - id: "JS-011"
+    name: "Use standard pagination object structure"
+    severity: "error"
+    description: |
+      All pages with tables MUST use the standard nested pagination object
+      structure. This ensures compatibility with the pagination macro and
+      consistent behavior across all pages.
+
+      REQUIRED structure:
+        pagination: {
+            page: 1,
+            per_page: 20,
+            total: 0,
+            pages: 0
+        }
+
+      WRONG (flat structure):
+        page: 1,
+        limit: 20,
+        total: 0,
+        skip: 0
+
+      WRONG (different property names):
+        pagination: {
+            currentPage: 1,
+            itemsPerPage: 20
+        }
+
+      Required computed properties:
+        - totalPages
+        - startIndex
+        - endIndex
+        - pageNumbers
+
+      Required methods:
+        - previousPage()
+        - nextPage()
+        - goToPage(pageNum)
+
+      Documentation: docs/frontend/shared/pagination.md
+    pattern:
+      file_pattern: "static/**/js/**/*.js"
+      required_in_pages_with_pagination:
+        - "pagination:"
+        - "pagination\\.page"
+        - "pagination\\.per_page"
+      anti_patterns_in_pagination_pages:
+        - "^\\s*page:\\s*\\d"
+        - "^\\s*limit:\\s*\\d"
+        - "^\\s*skip:\\s*"
+      exceptions:
+        - "init-alpine.js"
+
+  - id: "JS-012"
+    name: "Do not include /api/v1 prefix in API endpoints"
+    severity: "error"
+    description: |
+      When using apiClient.get(), apiClient.post(), etc., do NOT include
+      the /api/v1 prefix in the endpoint path. The apiClient automatically
+      prepends this prefix.
+
+      CORRECT:
+        apiClient.get('/admin/vendors')
+        apiClient.post('/admin/products')
+        const apiEndpoint = '/admin/vendors'
+
+      WRONG (causes double prefix /api/v1/api/v1/...):
+        apiClient.get('/api/v1/admin/vendors')
+        const apiEndpoint = '/api/v1/admin/vendors'
+
+      Exception: Direct fetch() calls without apiClient should use full path.
+
+      Documentation: docs/frontend/shared/api-client.md
+    pattern:
+      file_pattern: "static/**/js/**/*.js"
+      anti_patterns:
+        - "apiClient\\.(get|post|put|delete|patch)\\s*\\(\\s*['\"`]/api/v1"
+        - "apiEndpoint.*=.*['\"`]/api/v1"
+      exceptions:
+        - "init-api-client.js"
+
 # ============================================================================
 # TEMPLATE RULES (Jinja2)
 # ============================================================================