feat(nixserver): WHM-style collapsible category sidebar

Replace the flat 14-item nav with a full category sidebar matching
the WHM layout. 29 collapsible categories, ~210 menu items, live
search that filters across all categories and auto-expands results,
active route auto-expands the matching category, sidebar toggle
hides/shows the full panel.

server/package.json

@@ -1,6 +1,6 @@
 {
   "name": "@nixpanel/server",
-  "version": "0.4.1",
+  "version": "0.4.2",
   "private": true,
   "type": "module",
   "main": "dist/index.js",

server/src/app.ts

@@ -73,7 +73,7 @@ export async function buildApp() {
 
   fastify.get('/api/health', async () => ({
     status: 'ok',
-    version: '0.4.1',
+    version: '0.4.2',
     timestamp: new Date().toISOString(),
   }))