feat(web): metric-card widget and gauge widget redesign

[ZingerLittleBee]

Summary

• Add a new metric-card dashboard widget composed from header / value+delta / sparkline / stats subcomponents, with per-metric spec map, use-metric-series hook, config form, i18n strings, and widget-picker icon
• Redesign the gauge widget with an SVG ring and gradient stroke
• Add network and disk_io metric extractors so the new widget can render those series

Test plan

• bun run test (vitest) passes — includes the new metric-card render contract and gauge redesign tests
• bun x ultracite check clean
• bun run typecheck clean
• Visual check in the dashboard: add a metric-card for cpu / memory / network / disk_io, confirm value, delta sign, sparkline, and stats render correctly
• Visual check of the redesigned gauge widget (ring + gradient) across light/dark themes
• Widget picker shows the new metric-card icon and the config form saves/loads correctly