feat: add categories admin UI with CRUD

Wire up categories service, add CategoriesHandler with full CRUD, create index/edit templates, register routes in server.go, and add Categories nav link.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

views/categories/index.html

@@ -0,0 +1,35 @@
+<main class="container">
+  <div class="my-4 d-flex justify-content-between align-items-baseline">
+    <h4>Categories</h4>
+    <div>
+      <a href="/sites/{{ .site.ID }}/categories/new" class="btn btn-success">New Category</a>
+    </div>
+  </div>
+
+  <table class="table">
+    <thead>
+      <tr>
+        <th>Name</th>
+        <th>Slug</th>
+        <th>Posts</th>
+        <th></th>
+      </tr>
+    </thead>
+    <tbody>
+      {{ range .categories }}
+        <tr>
+          <td><a href="/sites/{{ $.site.ID }}/categories/{{ .ID }}">{{ .Name }}</a></td>
+          <td><code>{{ .Slug }}</code></td>
+          <td>{{ .PostCount }}</td>
+          <td>
+            <a href="/sites/{{ $.site.ID }}/categories/{{ .ID }}" class="btn btn-outline-secondary btn-sm">Edit</a>
+          </td>
+        </tr>
+      {{ else }}
+        <tr>
+          <td colspan="4" class="text-center text-muted py-4">No categories yet.</td>
+        </tr>
+      {{ end }}
+    </tbody>
+  </table>
+</main>