Add disconnect/uninstall for persistent sessions

- Server: Add DELETE /api/sessions/:id endpoint to disconnect agents
- Server: SessionManager.disconnect_session() sends Disconnect message
- Agent: Handle ADMIN_DISCONNECT to trigger uninstall
- Agent: Add startup::uninstall() to remove from startup and schedule exe deletion
- Dashboard: Add Disconnect button in Access tab machine details
- Dashboard: Add Chat button for persistent sessions

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

agent/Cargo.toml

@@ -73,6 +73,7 @@ windows = { version = "0.58", features = [
     "Win32_System_Threading",
     "Win32_System_Registry",
     "Win32_Security",
+    "Win32_Storage_FileSystem",
 ]}
 
 # Windows service support