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/AdminPromoGroupCreate.tsx

@@ -9,39 +9,7 @@ import {
   PromoGroupUpdateRequest,
 } from '../api/promocodes';
 import { AdminBackButton } from '../components/admin';
-
-// Icons
-const PlusIcon = () => (
-  <svg className="h-5 w-5" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-    <path strokeLinecap="round" strokeLinejoin="round" d="M12 4.5v15m7.5-7.5h-15" />
-  </svg>
-);
-
-const TrashIcon = () => (
-  <svg className="h-4 w-4" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-    <path
-      strokeLinecap="round"
-      strokeLinejoin="round"
-      d="M14.74 9l-.346 9m-4.788 0L9.26 9m9.968-3.21c.342.052.682.107 1.022.166m-1.022-.165L18.16 19.673a2.25 2.25 0 01-2.244 2.077H8.084a2.25 2.25 0 01-2.244-2.077L4.772 5.79m14.456 0a48.108 48.108 0 00-3.478-.397m-12 .562c.34-.059.68-.114 1.022-.165m0 0a48.11 48.11 0 013.478-.397m7.5 0v-.916c0-1.18-.91-2.164-2.09-2.201a51.964 51.964 0 00-3.32 0c-1.18.037-2.09 1.022-2.09 2.201v.916m7.5 0a48.667 48.667 0 00-7.5 0"
-    />
-  </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 { PlusIcon, RefreshIcon, TrashIcon } from '@/components/icons';
 
 interface PeriodDiscount {
   days: number | '';
@@ -340,7 +308,7 @@ export default function AdminPromoGroupCreate() {
                     onClick={() => removePeriodDiscount(index)}
                     className="p-1 text-dark-400 transition-colors hover:text-error-400"
                   >
-                    <TrashIcon />
+                    <TrashIcon className="h-4 w-4" />
                   </button>
                 </div>
               ))}
@@ -429,7 +397,7 @@ export default function AdminPromoGroupCreate() {
             disabled={!isValid || isLoading}
             className="btn-primary flex items-center gap-2"
           >
-            {isLoading && <RefreshIcon />}
+            {isLoading && <RefreshIcon spinning />}
             {isLoading ? t('admin.promoGroups.form.saving') : t('admin.promoGroups.form.save')}
           </button>
         </div>