[ENG-46317] feat(webkit): add Calendar component

[HerbertJulio]

Summary

Adds the Calendar component (inputs, monolithic) — an inline, always-visible date-picker surface for selecting a single date or a date range. Built spec-first (.specs/calendar.md), faithful to the Figma (node 5087-17541).

• v-model modelValue (Date | null single, { start, end } range), mode: 'single' | 'range', min, max, disabled, showHeader.
• Events: update:modelValue, month-change.
• Native Date only — no date library. 6×7 Sunday-start grid; roving-tabindex role="grid" with Arrow / Enter+Space / PageUp+PageDown keyboard nav.
• Styling is inline Tailwind on data-* variants (no JS class presets, no <style>); all tokens via var(--*) / DESIGN.md typography classes.
• Range highlight is a continuous band via a data-band model (single/start/middle/end).
• Storybook: Components/Inputs/Calendar (Default + Range).

Notes

• No new dependencies. No breaking changes.
• Layout/spacing/tokens match Figma 5087-17541: --bg-surface-raised panel, header pl-16/pr-12/py-8, grid p-12, 16px weekday→weeks gap, 4px between week rows, 36px day cells, 14px (text-body-sm) header/day type, 12px (text-label-sm) weekdays.
• One theme-gap recorded in the spec: Figma Typography/Heading/xss (14px regular) maps to .text-body-sm — naming-only gap, size/weight match.
• Out of scope (Figma optional sub-parts): the Date/Time input fields, "Apply Range" button, and timezone select below the grid.
• Related issue: ENG-46317.