[Design P2] Add sidebar network status indicator and 'Last Updated' timestamp

[AnilChinchawaleXDC]

Source: DESIGN-AUDIT.md

Problem: Missing network status indicator (e.g., 'XDC Mainnet • Block #99.2M • Online'). No 'Last Updated' timestamp at sidebar bottom.

Actions:

1. Add live network status indicator to sidebar
2. Show current block height and network health
3. Add 'Last Updated' relative timestamp at sidebar bottom
4. Update automatically via WebSocket or polling

Risk: Users lack context on data freshness and network state.

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Mirrored from Paperclip OPEA-371 (https://agi.openscan.ai/OPEA/issues/OPEA-371)