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/AdminLandingEditor.tsx

@@ -1,4 +1,5 @@
 import { useState, useCallback, useEffect, useRef, useMemo } from 'react';
+import { PiCaretDownDuotone } from 'react-icons/pi';
 import { useNavigate, useParams } from 'react-router';
 import { useQuery, useQueries, useMutation, useQueryClient } from '@tanstack/react-query';
 import { useTranslation } from 'react-i18next';
@@ -52,15 +53,7 @@ function isoToDatetimeLocal(iso: string): string {
 }
 
 const ChevronDownIcon = ({ open }: { open: boolean }) => (
-  <svg
-    className={cn('h-5 w-5 transition-transform', open && 'rotate-180')}
-    fill="none"
-    viewBox="0 0 24 24"
-    stroke="currentColor"
-    strokeWidth={2}
-  >
-    <path strokeLinecap="round" strokeLinejoin="round" d="M19.5 8.25l-7.5 7.5-7.5-7.5" />
-  </svg>
+  <PiCaretDownDuotone className={cn('h-5 w-5 transition-transform', open && 'rotate-180')} />
 );
 
 // ============ Collapsible Section ============