Add auth-menu fragment handler to account app

Phase 2 of fragment composition: account exposes auth-menu fragment
(sign-in button or user email for desktop + mobile) via
/internal/fragments/ endpoint. Updates shared submodule.

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

templates/fragments/auth_menu.html

@@ -0,0 +1,36 @@
+{# Desktop auth menu #}
+<span id="auth-menu-desktop">
+{% if user_email %}
+  <a
+    href="{{ account_url('/') }}"
+    class="justify-center cursor-pointer flex flex-row items-center p-3 gap-2 rounded bg-stone-200 text-black {{ select_colours }}"
+    data-close-details
+  >
+    <i class="fa-solid fa-user"></i>
+    <span>{{ user_email }}</span>
+  </a>
+{% else %}
+  <a
+    href="{{ account_url('/') }}"
+    class="justify-center cursor-pointer flex flex-row items-center p-3 gap-2 rounded bg-stone-200 text-black {{ select_colours }}"
+    data-close-details
+  >
+    <i class="fa-solid fa-key"></i>
+    <span>sign in or register</span>
+  </a>
+{% endif %}
+</span>
+{# Mobile auth menu #}
+<span id="auth-menu-mobile">
+{% if user_email %}
+  <a href="{{ account_url('/') }}" data-close-details>
+    <i class="fa-solid fa-user"></i>
+    <span>{{ user_email }}</span>
+  </a>
+{% else %}
+  <a href="{{ account_url('/') }}">
+    <i class="fa-solid fa-key"></i>
+    <span>sign in or register</span>
+  </a>
+{% endif %}
+</span>