feat(cabinet): migrate all icons to the panel's Phosphor set

Replace the cabinet's hand-written heroicons-style SVG icon components with
the Remnawave panel's own icon family — Phosphor via react-icons/pi (Duotone).
All icons now live in a single central barrel (src/components/icons:
index + extended-icons + editor-icons, 144 icons); every feature file imports
from it instead of redefining inline SVGs (~440 icon defs removed).

- Add react-icons dependency
- Desktop nav (AppShell) now uses the central Phosphor icons, removing a
  stroke-vs-duotone inconsistency the partial migration introduced
- Icons with custom props (SortIcon's direction, expandable chevrons) kept as
  thin Phosphor wrappers; the RemnawaveIcon brand logo and animated SVGs
  (checkmarks, spinners) are intentionally left as-is
- Restore the original per-call-site icon sizes wherever the centralized
  default differed (211 call sites)

src/pages/AdminTickets.tsx

@@ -9,6 +9,7 @@ import { adminApi, AdminTicket, AdminTicketDetail } from '../api/admin';
 import { ticketsApi } from '../api/tickets';
 import { copyToClipboard as copyText } from '../utils/clipboard';
 import { usePlatform } from '../platform/hooks/usePlatform';
+import { BackIcon } from '@/components/icons';
 
 interface MediaAttachment {
   id: string;
@@ -39,19 +40,6 @@ const ALLOWED_FILE_TYPES: Record<string, string> = {
 const ACCEPT_STRING = Object.keys(ALLOWED_FILE_TYPES).join(',');
 const MAX_FILE_SIZE = 10 * 1024 * 1024; // 10MB
 
-// BackIcon
-const BackIcon = () => (
-  <svg
-    className="h-5 w-5 text-dark-400"
-    fill="none"
-    viewBox="0 0 24 24"
-    stroke="currentColor"
-    strokeWidth={2}
-  >
-    <path strokeLinecap="round" strokeLinejoin="round" d="M15.75 19.5L8.25 12l7.5-7.5" />
-  </svg>
-);
-
 export default function AdminTickets() {
   const { t } = useTranslation();
   const navigate = useNavigate();