[Design P2] Add sparklines and trend indicators to dashboard node cards

[AnilChinchawaleXDC]

Source: DESIGN-AUDIT.md

Problem: No sparklines on main dashboard node cards. No trend indicators on stat boxes.

Actions:

1. Add inline SVG sparklines for block height trend
2. Add up/down trend arrows to stat boxes
3. Use historical metrics data for sparkline generation
4. Ensure sparklines are lightweight (SVG)

Risk: Users miss performance trends at a glance.

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Mirrored from Paperclip OPEA-372 (https://agi.openscan.ai/OPEA/issues/OPEA-372)