Hide dashboard sidebar on staff/supervisor/admin sub-pages; fix mobile overflow

- Sidebar now shows only on each role's root landing page
  (/dashboard/staff, /dashboard/supervisor, /dashboard/admin) and hides on
  every sub-page beneath them, not just Reports. Navbar's "Dashboard" link
  always leads back to the role root.
- Fixed two dropdown panels (EventsDropdown, the Email/WhatsApp Attendees
  recipient picker) that could extend past the viewport's right edge on
  narrow screens; both now stretch to their trigger's width like other
  dropdowns in the app already do.
- Wrapped the Admin Cashup event-costs table in the same overflow-auto
  container every sibling table on that page already uses, so it scrolls
  horizontally on narrow screens instead of squeezing its columns.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/app/dashboard/supervisor/whatsapp-attendees/page.tsx

@@ -86,7 +86,7 @@ function AttendeesCheckboxDropdown({
         )}
       </button>
       {open && (
-        <div className="absolute z-10 mt-1 w-64 max-h-72 overflow-auto bg-white border rounded shadow">
+        <div className="absolute z-10 mt-1 left-0 right-0 max-h-72 overflow-auto bg-white border rounded shadow">
           <div className="px-3 py-2 border-b sticky top-0 bg-white space-y-1">
             <label className="text-sm flex items-center gap-2">
               <input type="checkbox" checked={allSelected} onChange={(e) => toggleAll(e.target.checked)} />