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>

app/templates/admin/inventory.html

@@ -1,7 +1,7 @@
 {# app/templates/admin/inventory.html #}
 {% extends "admin/base.html" %}
 {% from 'shared/macros/pagination.html' import pagination %}
-{% from 'shared/macros/headers.html' import page_header %}
+{% from 'shared/macros/headers.html' import page_header_flex, refresh_button %}
 {% from 'shared/macros/alerts.html' import loading_state, error_state %}
 {% from 'shared/macros/tables.html' import table_wrapper %}
 {% from 'shared/macros/modals.html' import modal_simple %}
@@ -12,7 +12,38 @@
 {% block alpine_data %}adminInventory(){% endblock %}
 
 {% block content %}
-{{ page_header('Inventory', subtitle='Manage stock levels across all vendors') }}
+<!-- Page Header with Vendor Selector -->
+{% call page_header_flex(title='Inventory', subtitle='Manage stock levels across all vendors') %}
+    <div class="flex items-center gap-4">
+        <!-- Vendor Autocomplete (Tom Select) -->
+        {{ vendor_selector(
+            ref_name='vendorSelect',
+            id='inventory-vendor-select',
+            placeholder='Filter by vendor...',
+            width='w-80'
+        ) }}
+        {{ refresh_button(loading_var='loading', onclick='refresh()', variant='secondary') }}
+    </div>
+{% endcall %}
+
+<!-- Selected Vendor Info -->
+<div x-show="selectedVendor" x-transition class="mb-6 p-3 bg-purple-50 dark:bg-purple-900/20 rounded-lg border border-purple-200 dark:border-purple-800">
+    <div class="flex items-center justify-between">
+        <div class="flex items-center gap-3">
+            <div class="w-8 h-8 rounded-full bg-purple-100 dark:bg-purple-900 flex items-center justify-center">
+                <span class="text-sm font-semibold text-purple-600 dark:text-purple-300" x-text="selectedVendor?.name?.charAt(0).toUpperCase()"></span>
+            </div>
+            <div>
+                <span class="font-medium text-purple-800 dark:text-purple-200" x-text="selectedVendor?.name"></span>
+                <span class="ml-2 text-xs text-purple-600 dark:text-purple-400 font-mono" x-text="selectedVendor?.vendor_code"></span>
+            </div>
+        </div>
+        <button @click="clearVendorFilter()" class="text-purple-600 dark:text-purple-400 hover:text-purple-800 dark:hover:text-purple-200 text-sm flex items-center gap-1">
+            <span x-html="$icon('x', 'w-4 h-4')"></span>
+            Clear filter
+        </button>
+    </div>
+</div>
 
 {{ loading_state('Loading inventory...') }}
 
@@ -102,14 +133,6 @@
 
         <!-- Filters -->
         <div class="flex flex-wrap gap-3">
-            <!-- Vendor Filter (Tom Select) -->
-            {{ vendor_selector(
-                ref_name='vendorSelect',
-                id='inventory-vendor-select',
-                placeholder='Filter by vendor...',
-                width='w-64'
-            ) }}
-
             <!-- Location Filter -->
             <select
                 x-model="filters.location"