feat(admin): add Platform Administration section to sidebar

Reorganized sidebar navigation:
- Added "Platform Administration" section header below Dashboard
- Grouped Companies, Vendors, Users, Customers, Marketplace under it
- Added new Customers page (empty placeholder for future)
- Changed Marketplace icon from shopping-bag to globe
- Renamed "Marketplace Import" to "Marketplace" for brevity

New files:
- app/templates/admin/customers.html
- Route: GET /admin/customers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/routes/admin_pages.py

@@ -23,6 +23,7 @@ Routes:
 - GET /vendor-themes → Vendor themes selection page (auth required)
 - GET /vendors/{vendor_code}/theme → Vendor theme editor (auth required)
 - GET /users         → User management page (auth required)
+- GET /customers     → Customer management page (auth required)
 - GET /imports       → Import history page (auth required)
 - GET /settings      → Settings page (auth required)
 - GET /platform-homepage → Platform homepage manager (auth required)
@@ -449,6 +450,30 @@ async def admin_user_edit_page(
     )
 
 
+# ============================================================================
+# CUSTOMER MANAGEMENT ROUTES
+# ============================================================================
+
+
+@router.get("/customers", response_class=HTMLResponse, include_in_schema=False)
+async def admin_customers_page(
+    request: Request,
+    current_user: User = Depends(get_current_admin_from_cookie_or_header),
+    db: Session = Depends(get_db),
+):
+    """
+    Render customers management page.
+    Shows list of all platform customers.
+    """
+    return templates.TemplateResponse(
+        "admin/customers.html",
+        {
+            "request": request,
+            "user": current_user,
+        },
+    )
+
+
 # ============================================================================
 # IMPORT MANAGEMENT ROUTES
 # ============================================================================