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/partials/page-tabs.html

@@ -0,0 +1,21 @@
+{{define "page-tabs.html"}}
+{{range .Pages}}
+<button hx-get="/pages/{{.ID}}" 
+        hx-target="#widget-grid" 
+        hx-swap="innerHTML"
+        hx-push-url="false"
+        data-page-id="{{.ID}}"
+        class="page-tab px-4 py-2 text-sm font-medium text-gray-600 hover:text-gray-800 border-b-2 border-transparent hover:border-gray-300 transition-colors">
+    {{.Name}}
+</button>
+{{end}}
+<button hx-get="/pages/new-form" 
+        hx-target="#page-form-modal" 
+        hx-swap="innerHTML"
+        class="px-4 py-2 text-sm font-medium text-gray-400 hover:text-gray-600 transition-colors"
+        title="Add new page">
+    <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>
+{{end}}