Complete tasks 4.1-4.2: Page management service and HTTP endpoints

- Implemented PageService with full CRUD operations
- Added GetPages, CreatePage, UpdatePage, DeletePage, ReorderPages methods
- Cascade deletion of widgets when page is deleted
- Prevention of last page deletion
- Created page HTTP endpoints (GET, POST, PUT, DELETE, reorder)
- HTMX-friendly HTML fragment responses
- Comprehensive unit tests for service and handlers
- Updated dashboard to use PageService and create default pages

templates/dashboard.html

@@ -48,19 +48,7 @@
             
             <!-- Page Tabs -->
             <div id="page-tabs" class="flex space-x-1 -mb-px">
-                {{range .Pages}}
-                <button hx-get="/pages/{{.ID}}" 
-                        hx-target="#widget-grid" 
-                        hx-swap="innerHTML"
-                        class="px-4 py-2 text-sm font-medium {{if .Active}}border-b-2 border-blue-500 text-blue-600{{else}}text-gray-600 hover:text-gray-800{{end}}">
-                    {{.Name}}
-                </button>
-                {{end}}
-                <button class="px-4 py-2 text-sm font-medium text-gray-400 hover:text-gray-600">
-                    <svg class="w-4 h-4" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-                        <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4"/>
-                    </svg>
-                </button>
+                {{template "page-tabs.html" .}}
             </div>
         </div>
     </header>