feat(core): add all-in-one MetricCard component

[FrancescoCastaldi]

Description

This PR introduces the <MetricCard> (metric_card) all-in-one KPI card component.

Modern dashboards frequently need compact, highly readable summary cards that unify the primary metric figure, comparison/trend badges, and sparkline history in a clean card container.

Features

1. Primary KPI Value:
   • Formats values using Evidence format strings (value_fmt="usd", value_fmt="pct").
   • Shows spinner during query loading.
2. Integrated Sparkline:
   • sparkline_date="date_col", sparkline_value="val_col", sparkline_type="line|area|bar".
   • Embeds interactive micro-charts directly on the bottom right of the card.
3. Comparison & Delta Badges:
   • Automatic delta calculation (comparison="prior_col" or explicit delta/values).
   • Renders trend pill badges (badge=true) or plain deltas with down_is_good awareness.
4. Interactive Navigation:
   • Optional link="/details" turns the entire card into an interactive card button with hover elevation.
5. Unit Tests:
   • Added MetricCard.test.ts.

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[FrancescoCastaldi]

Hi @archiewood @hughess! 👋 This PR introduces the all-in-one KPI component combining BigValue, Sparkline trend, and Delta badge. Whenever you get a chance, we would love your review!

[hughess]

Hi @FrancescoCastaldi - why not use (or adjust) big_value for this use case?

[FrancescoCastaldi]

Good point @hughess! Refactored to build directly on top of �ig_value instead of introducing a separate tag.

We have removed the standalone metric_card and extended �ig_value to support:

1. subtitle (and subtitle_class): secondary context label beneath the title.
2. comparison.chip: render delta comparisons as styled pill badges (chip=true).
3. card: optional standalone card styling (card=true) with rounded borders, shadow, and background padding.

All tests updated and passing! 🚀