feat(dashboard): admin "Beheer" section linking the admin pages

Add a capability-gated Beheer section to the dashboard listing the admin pages
(Huisstijl, Stamdata, Aanvragen), each shown when the principal holds its
capability — so admin pages are discoverable, not URL-only. Extract the admin
link list to shared/layout/admin-links.ts as one source of truth, reused by the
site header (was ADMIN_NAV_ITEMS) and the new section. Capability-gated, never
role-derived (PRD-0002).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/locale/messages.en.xlf

@@ -3642,6 +3642,22 @@
         <source>Aanvragen</source>
         <target datatype="html">Cases</target>
       </trans-unit>
+      <trans-unit id="dashboard.beheer" datatype="html">
+        <source>Beheer</source>
+        <target datatype="html">Administration</target>
+      </trans-unit>
+      <trans-unit id="admin.link.huisstijl.desc" datatype="html">
+        <source>Organisatiesjablonen voor brieven beheren</source>
+        <target datatype="html">Manage organisation letter templates</target>
+      </trans-unit>
+      <trans-unit id="admin.link.stamdata.desc" datatype="html">
+        <source>Business-tabellen onderhouden</source>
+        <target datatype="html">Maintain business reference tables</target>
+      </trans-unit>
+      <trans-unit id="admin.link.zaken.desc" datatype="html">
+        <source>Alle aanvragen bekijken en beheren</source>
+        <target datatype="html">View and manage all cases</target>
+      </trans-unit>
       <trans-unit id="adminCases.heading" datatype="html">
         <source>Aanvragen beheren</source>
         <target datatype="html">Manage cases</target>