Add macOS and Windows agents + multi-platform install UI

- agent/skywatch-agent-macos.sh: Full macOS bash agent (launchd)
  - CPU via top -l 2, memory via vm_stat/sysctl, disk via df -P
  - Network via netstat -ib, uptime via kern.boottime, OS via sw_vers
- agent/install-agent-macos.sh: Installs macOS agent as launchd daemon
- agent/skywatch-agent-windows.ps1: PowerShell agent (Task Scheduler)
  - CPU via Win32_PerfRawData two-sample delta, memory via Win32_OperatingSystem
  - Disk via Get-PSDrive, network via Get-NetAdapterStatistics
- agent/install-agent-windows.ps1: Installs Windows agent via Task Scheduler
- server/index.js: New routes for macOS and Windows agent scripts/installers
- client/src/Agents.js: Multi-platform install panel with Linux/macOS/Windows tabs
- client/src/Agents.css: Platform tab button styles

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/build/asset-manifest.json

@@ -1,15 +1,15 @@
 {
   "files": {
-    "main.css": "/static/css/main.e143c118.css",
-    "main.js": "/static/js/main.a720143a.js",
+    "main.css": "/static/css/main.4668db54.css",
+    "main.js": "/static/js/main.f791418f.js",
     "static/js/453.d456ce70.chunk.js": "/static/js/453.d456ce70.chunk.js",
     "index.html": "/index.html",
-    "main.e143c118.css.map": "/static/css/main.e143c118.css.map",
-    "main.a720143a.js.map": "/static/js/main.a720143a.js.map",
+    "main.4668db54.css.map": "/static/css/main.4668db54.css.map",
+    "main.f791418f.js.map": "/static/js/main.f791418f.js.map",
     "453.d456ce70.chunk.js.map": "/static/js/453.d456ce70.chunk.js.map"
   },
   "entrypoints": [
-    "static/css/main.e143c118.css",
-    "static/js/main.a720143a.js"
+    "static/css/main.4668db54.css",
+    "static/js/main.f791418f.js"
   ]
 }