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/components/connection/InstallationGuide.tsx

@@ -12,6 +12,7 @@ import { useTheme } from '@/hooks/useTheme';
 import { CardsBlock, TimelineBlock, AccordionBlock, MinimalBlock, BlockButtons } from './blocks';
 import type { BlockRendererProps } from './blocks';
 import TvQuickConnect from './TvQuickConnect';
+import { BackIcon } from '@/components/icons';
 
 const platformOrder = ['ios', 'android', 'windows', 'macos', 'linux', 'androidTV', 'appleTV'];
 
@@ -33,12 +34,6 @@ const RENDERERS: Record<string, React.ComponentType<BlockRendererProps>> = {
   minimal: MinimalBlock,
 };
 
-const BackIcon = () => (
-  <svg className="h-6 w-6" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
-    <path strokeLinecap="round" strokeLinejoin="round" d="M15 19l-7-7 7-7" />
-  </svg>
-);
-
 interface Props {
   appConfig: AppConfig;
   onOpenDeepLink: (url: string) => void;
@@ -200,7 +195,7 @@ export default function InstallationGuide({
             aria-label={t('common.back', 'Back')}
             className="flex h-10 w-10 items-center justify-center rounded-xl border border-dark-700 bg-dark-800 transition-colors hover:border-dark-600"
           >
-            <BackIcon />
+            <BackIcon className="h-6 w-6" />
           </button>
         )}
         <h2 className="flex-1 text-lg font-bold text-dark-100">