feat: add system gateway restart control

[HexaField]

Summary

• add an OpenClaw gateway restart control to System → Devices
• add a server endpoint that executes openclaw gateway restart via execFile
• prevent duplicate restart requests and surface restart/reconnect status in the UI
• add server and client coverage plus an implementation spec

Verification

• pnpm exec vitest run packages/client/src/features/system/DevicesTab.test.ts packages/server/src/system/system.test.ts
• pnpm --filter @sovereign/server check
• note: repo-wide client typecheck is currently blocked by pre-existing WorkspaceView.tsx errors on main, unrelated to this change