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-tab.html

@@ -0,0 +1,10 @@
+{{define "page-tab.html"}}
+<button hx-get="/pages/{{.Page.ID}}" 
+        hx-target="#widget-grid" 
+        hx-swap="innerHTML"
+        hx-push-url="false"
+        data-page-id="{{.Page.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">
+    {{.Page.Name}}
+</button>
+{{end}}