Add site-grouped device list in sidebar

Fetch device sites from /api/v1/deviceSites in parallel with devices,
then group cameras by site with collapsible headers. Search now also
matches site names. Falls back to flat list if sites API is unavailable.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

preload.js

@@ -4,6 +4,7 @@ const { contextBridge, ipcRenderer } = require('electron');
 // the ipcRenderer without exposing the entire object
 contextBridge.exposeInMainWorld('electronAPI', {
   getDevices: (params) => ipcRenderer.invoke('api-get-devices', params),
+  getDeviceSites: (params) => ipcRenderer.invoke('api-get-device-sites', params),
   getAuthInfo: (params) => ipcRenderer.invoke('api-get-auth-info', params),
 
   // Camera proxy functionality