feat(tasks): add calendar view for tasks (month / week / day)

[paryanineil]

Summary

Adds a Calendar view to the Tasks page, alongside the existing List and Kanban views. Tasks are laid out by their due_date, with Month, Week, and Day sub-modes.

What's included

• TaskCalendar component (frontend/src/components/TaskCalendar.tsx)
  • Month — full 6-week grid, status-colored task chips, +N more overflow per day
  • Week — 7 day-columns listing each day's tasks
  • Day — single-day agenda with status + priority
  • Mode-aware prev / next / Today navigation and today highlighting
  • A "No due date" tray so tasks without a due date are never hidden
  • Built with date-fns (already a dependency) and shadcn Button / Badge
• View switcher — calendar toggle added next to List/Kanban on the Tasks page
• Saveable view — adds calendar to Hive View.view_type; the mode is restored from saved views and the sidebar

Notes

• Additive only — no changes to existing List/Kanban behavior and no backend API changes. The single backend change is a new option on the existing Hive View.view_type Select field (list → list\nkanban\ncalendar).
• Existing task filters (status / priority / project / assignee) apply to the calendar, same as the other views.
• Clicking a task opens the existing Task Details sheet.

Testing

• yarn build passes, tsc --noEmit clean, ESLint clean on the new component
• Verified in-browser against a seeded dataset: tasks render on their due dates across all three modes, navigation + today highlight work, clicking a task opens its detail sheet, and undated tasks appear in the tray. No console errors.