feat: add data-velxio-slot markers for overlay portal targets

Three small markers (each one HTML attribute) so private overlays can
portal-inject UI into well-defined places without forking the upstream
component:

- AppHeader user dropdown: data-velxio-slot="user-menu"
  Lets overlays add menu items between "My projects" and "Sign out"
  (e.g. a Privacy / opt-out item).
- AdminPage tab bar: data-velxio-slot="admin-tabs"
  Lets overlays add extra tabs alongside Dashboard / Users / Projects /
  Boards (e.g. a Pro Analytics tab).
- AdminPage tab content area: data-velxio-slot="admin-tab-content"
  Sibling div where overlay tab content can portal-render.

Generic markers, no overlay-specific code in upstream. Anyone with
private extensions can use them. The OSS build is otherwise unchanged.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/AppHeader.tsx

@@ -264,6 +264,7 @@ export const AppHeader: React.FC<AppHeaderProps> = ({ autoSave }) => {
 
               {dropdownOpen && (
                 <div
+                  data-velxio-slot="user-menu"
                   style={{
                     position: 'absolute',
                     right: 0,

frontend/src/pages/AdminPage.tsx

@@ -625,7 +625,7 @@ function AdminDashboard() {
         </div>
       </div>
 
-      <div style={s.tabs}>
+      <div data-velxio-slot="admin-tabs" style={s.tabs}>
         <button
           style={tab === 'dashboard' ? s.tabActive : s.tabBtn}
           onClick={() => setTab('dashboard')}
@@ -648,6 +648,7 @@ function AdminDashboard() {
           Boards
         </button>
       </div>
+      <div data-velxio-slot="admin-tab-content" />
 
       {tab === 'dashboard' && (
         <div style={s.tabContent}>