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/AdminPromocodeCreate.tsx

@@ -13,35 +13,7 @@ import {
 } from '../api/promocodes';
 import { tariffsApi } from '../api/tariffs';
 import { usePlatform } from '../platform/hooks/usePlatform';
-
-// Icons
-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>
-);
-
-const RefreshIcon = () => (
-  <svg
-    className="h-4 w-4 animate-spin"
-    fill="none"
-    viewBox="0 0 24 24"
-    stroke="currentColor"
-    strokeWidth={2}
-  >
-    <path
-      strokeLinecap="round"
-      strokeLinejoin="round"
-      d="M16.023 9.348h4.992v-.001M2.985 19.644v-4.992m0 0h4.992m-4.993 0l3.181 3.183a8.25 8.25 0 0013.803-3.7M4.031 9.865a8.25 8.25 0 0113.803-3.7l3.181 3.182m0-4.991v4.99"
-    />
-  </svg>
-);
+import { BackIcon, RefreshIcon } from '@/components/icons';
 
 export default function AdminPromocodeCreate() {
   const { t } = useTranslation();
@@ -557,7 +529,7 @@ export default function AdminPromocodeCreate() {
             disabled={!isValid() || isLoading}
             className="btn-primary flex items-center gap-2"
           >
-            {isLoading && <RefreshIcon />}
+            {isLoading && <RefreshIcon spinning />}
             {isLoading ? t('admin.promocodes.form.saving') : t('admin.promocodes.form.save')}
           </button>
         </div>