[Integration] useDate system composable integration

[johnleider]

Summary

Integrate useDate with the v0 plugin system and expand adapter coverage to match real-world usage.

Integration Points

• Accept configuration from unified v0 plugin options
• Use useLocale for locale-aware date formatting
• Respect useLocale RTL for date display order
• Cache adapter instance configuration
• Add date-fns adapter
• Add Luxon adapter
• Add Day.js adapter (nice-to-have)

Out of Scope (Vuetify-level concerns)

• Moment.js adapter (deprecated — Day.js covers legacy users)
• js-joda adapter (niche)
• useDateFormat() composable (opinionated input parsing)
• weekInfo() locale lookup table (could be separate data package)
• Custom format function support
• Module augmentation for type overrides

Related

• [Bug] useDate - getDiff should be symmetrical #100 - Bug: getDiff should be symmetrical
• [Audit] useDate - ensure adapter system matches Vuetify #106 - Audit useDate adapter system
• [Integration] Convert Vuetify to use @vuetify/0 useDate #111 - Convert Vuetify to use v0 useDate
• Full comparison: specs/composables/useDate-comparison.md