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/AdminTrafficUsage.tsx

@@ -768,7 +768,7 @@ export default function AdminTrafficUsage() {
 
           {/* Threshold inputs */}
           <div className="flex items-center gap-1.5 rounded-lg border border-dark-700 bg-dark-800 px-2 py-1">
-            <ShieldIcon />
+            <ShieldIcon className="h-3.5 w-3.5" />
             <input
               type="number"
               value={totalThreshold}
@@ -784,12 +784,12 @@ export default function AdminTrafficUsage() {
                 onClick={() => setTotalThreshold('')}
                 className="text-dark-500 hover:text-dark-300"
               >
-                <XIcon />
+                <XIcon className="h-3 w-3" />
               </button>
             )}
           </div>
           <div className="flex items-center gap-1.5 rounded-lg border border-dark-700 bg-dark-800 px-2 py-1">
-            <ServerSmallIcon />
+            <ServerSmallIcon className="h-3.5 w-3.5" />
             <input
               type="number"
               value={nodeThreshold}
@@ -805,7 +805,7 @@ export default function AdminTrafficUsage() {
                 onClick={() => setNodeThreshold('')}
                 className="text-dark-500 hover:text-dark-300"
               >
-                <XIcon />
+                <XIcon className="h-3 w-3" />
               </button>
             )}
           </div>