feat(Calendar): add month and year selection (upstream nuxt/ui#6582)

[IgorShevchik]

Context

Upstream nuxt/ui shipped ca3c72eb — feat(Calendar): add month and year selection (nuxt/ui#6582) — the largest single component change in the current sync stream:

• src/runtime/components/Calendar.vue +296 lines
• src/theme/calendar.ts +102 lines
• docs/content/docs/2.components/calendar.md +226 lines, playgrounds/nuxt/app/pages/components/calendar.vue, a new CalendarTodayExample.vue
• ~4,800-line snapshot regen per project

It adds a type prop (date | month | year), a drill-up/down view state machine (day → month → year), a viewControl heading button that cycles views, month/year picker grids via reka-ui's namespaced MonthPicker / YearPicker (+ range variants), new emits (update:placeholder, update:startValue, update:validModelValue), and new slots (month-cell, year-cell, and an extended heading signature).

Decision in the sync stream — SKIPPED (no-op)

Recorded as a skip in the ledger (cursor advanced, no source change) per maintainer decision. Rationale:

• The component logic ports mechanically, but the theme does not: upstream's calendar.ts is built on options.theme.colors loops + nuxt/ui semantic tokens (bg-${color}, text-inverted, ring-accented, text-muted, data-selected, …). b24ui's calendar.ts is air-design-token based and has no theme.colors, so the new view variant + daySizes/pickerSizes compounds + month/year cellTrigger styling + headingLabel slot need a deliberate air-design pass (how month/year cells and the viewControl button look in b24ui), not a 1:1 translation.
• The size + design-decision content warranted handling it as its own task rather than a rushed mega-PR.

reka-ui 2.9.10 (current in b24ui) does expose MonthPicker / YearPicker / MonthRangePicker / YearRangePicker namespaced, so there's no upstream-dependency blocker.

Scope of this issue (port the feature, adapted to air design)

• Calendar.vue: port the view-state machine and template (Picker.*, month/year grids, viewControl heading button, cellProps, new emits/slots), adapted to b24ui (B24Button color="air-tertiary", b24ui prop, IconComponent icons via dictionary/icons, btnSize).
• theme/calendar.ts: design + apply the view variant (day/month/year), the size→view-size compounds (daySizes/pickerSizes equivalents), headingLabel slot, and month/year cellTrigger styling in air tokens consistent with the existing day cells (b24ui has no theme.colors, so adapt the per-color compounds to b24ui's single air color model).
• docs calendar.md + CalendarTodayExample.vue, playground calendar.vue (B24-namespaced, air examples).
• Regenerate Calendar snapshots (nuxt + vue).

References

• Upstream commit: ca3c72eb6606da882a6aba5c9f35a3ff0dddf820 (feat(Calendar): add month and year selection nuxt/ui#6582)
• Sync ledger: .sync/nuxt-ui.json → ca3c72eb… entry (decision: skip)
• reka-ui pickers confirmed available at 2.9.10

No behaviour change has been made by the sync stream; this is deferred to a focused, design-aware implementation.