docs(admin): new screenshots for Import page and hamburger sidebar

- Replace p9a_admin_sync.png with p9a_admin_import.png (new Import page)
- Add p9a_admin_sidebar.png (mobile sidebar with hamburger open)
- Update ADMIN_GUIDE.md with mobile navigation section
- Add screenshot reference in Import section

docs/ADMIN_GUIDE.md

@@ -48,6 +48,12 @@ After logging in, the dashboard shows:
 
 ![Admin Dashboard](images/p9a_admin_dashboard.png)
 
+### Mobile Navigation
+
+On phones, the sidebar is hidden behind a **☰ hamburger button**. Tap it to slide the navigation in from the left. The sidebar auto-closes when you tap a nav item or tap the dark overlay behind it.
+
+![Sidebar on Mobile](images/p9a_admin_sidebar.png)
+
 ### Sidebar Navigation
 
 The sidebar on the left provides access to:
@@ -129,7 +135,9 @@ Manage these lookup tables:
 
 Navigate to **📥 Import** in the sidebar.
 
-The Import page lets you upload asset data from Excel files (Cantaloupe exports or other structured spreadsheets).
+![Excel Import](images/p9a_admin_import.png)
+
+The Import page lets you upload asset data from Excel files.
 
 ### How it Works