[Dashboard] Minimal client with WebSocket

[varev-dev]

Goal

Deliver a minimal dashboard client that connects to the server via WebSocket and displays live metric updates for a selected agent.

Acceptance criteria

• Client can open a WebSocket connection to configured backend URL and receive messages in real time.
• Dashboard visibly shows current connection state and updates it after disconnect/reconnect.
• For valid incoming metric messages, UI updates latest displayed values within 1 second from message arrival.
• On temporary connection loss, client attempts reconnect automatically without page refresh.
• On malformed message, client does not crash; error is logged and UI remains usable.
• If no data is received for a configured timeout (e.g. 10s), UI shows stale data indicator.
• Basic manual test scenario is documented (how to run client, connect, simulate disconnect, verify reconnect).

Sub-tasks & Scope of work

• Dashboard UI: Create a basic SPA view with connection status, selected agent_id, and live list/cards for core metrics (CPU, memory, disk, network).
• WebSocket Module: Implement client with configurable URL (from env), connect/disconnect lifecycle, payload validation, and automatic reconnect with backoff (1s → 2s → 5s, capped).
• State Management: Add minimal state handling for latest metric values and last update timestamp.
• Error Handling: Handle error states in UI gracefully (server unavailable, invalid payload, timeout).
• Documentation: Add developer documentation note with test steps and known limitations for running/testing WS mode.