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

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+{{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}}

templates/partials/page-tabs.html

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+{{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}}

templates/partials/widget-grid.html

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+{{define "widget-grid.html"}}
+{{if .Widgets}}
+    {{range .Widgets}}
+    <div class="widget bg-white rounded-lg shadow-sm border border-gray-200 p-4" data-widget-id="{{.ID}}">
+        <div class="widget-handle cursor-move mb-2 flex items-center justify-between">
+            <h3 class="text-lg font-semibold text-gray-800">
+                {{if .Title}}{{.Title}}{{else}}{{.Type}} Widget{{end}}
+            </h3>
+            <button hx-delete="/widgets/{{.ID}}" 
+                    hx-target="closest .widget" 
+                    hx-swap="outerHTML swap:1s"
+                    hx-confirm="Are you sure you want to delete this widget?"
+                    class="text-gray-400 hover:text-red-600 transition-colors">
+                <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M6 18L18 6M6 6l12 12"/>
+                </svg>
+            </button>
+        </div>
+        <div class="widget-content">
+            <!-- Widget-specific content will be loaded here -->
+            <p class="text-gray-500 text-sm">{{.Type}} widget content</p>
+        </div>
+    </div>
+    {{end}}
+{{else}}
+    <div class="col-span-full text-center text-gray-500 py-12">
+        <svg class="w-16 h-16 mx-auto mb-4 text-gray-300" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M20 13V6a2 2 0 00-2-2H6a2 2 0 00-2 2v7m16 0v5a2 2 0 01-2 2H6a2 2 0 01-2-2v-5m16 0h-2.586a1 1 0 00-.707.293l-2.414 2.414a1 1 0 01-.707.293h-3.172a1 1 0 01-.707-.293l-2.414-2.414A1 1 0 006.586 13H4"/>
+        </svg>
+        <p class="text-lg font-medium mb-2">No widgets yet</p>
+        <p class="text-sm">Click the + button to add your first widget</p>
+    </div>
+{{end}}
+{{end}}