feat: add platform selection frontend for platform admins

Frontend implementation of platform admin flow:
- Update login.js to check for platform selection after login
- Add platform selection page (/admin/select-platform)
- Add platform context indicator in admin header
- Add is_super_admin to UserResponse schema
- Show "Super Admin" badge or platform name with switch option

Platform admins now:
1. Login normally at /admin/login
2. Get redirected to /admin/select-platform if they have multiple platforms
3. See current platform in header with option to switch

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/routes/admin_pages.py

@@ -95,6 +95,32 @@ async def admin_login_page(
     return templates.TemplateResponse("admin/login.html", {"request": request})
 
 
+@router.get("/select-platform", response_class=HTMLResponse, include_in_schema=False)
+async def admin_select_platform_page(
+    request: Request,
+    current_user: User | None = Depends(get_current_admin_optional),
+):
+    """
+    Render platform selection page for platform admins.
+
+    Platform admins with access to multiple platforms must select
+    which platform they want to manage before accessing the dashboard.
+    Super admins are redirected to dashboard (they have global access).
+    """
+    if not current_user:
+        # Not logged in, redirect to login
+        return RedirectResponse(url="/admin/login", status_code=302)
+
+    if current_user.is_super_admin:
+        # Super admins don't need platform selection
+        return RedirectResponse(url="/admin/dashboard", status_code=302)
+
+    return templates.TemplateResponse(
+        "admin/select-platform.html",
+        {"request": request, "user": current_user},
+    )
+
+
 # ============================================================================
 # AUTHENTICATED ROUTES (Admin Only)
 # ============================================================================