Design and add a Kanban planning view to the PyQt workspace

[ZFordDev]

Goal

Add a Kanban view that complements the existing Tasks and Calendar views for active planning without turning Lite or CLI into complex board interfaces.

Design decisions

• Define whether columns represent workflow status, user-defined stages, or a small fixed default set.
• Decide how existing completion state, categories, priorities, dates, recurrence, and reminders appear on cards.
• Define drag-and-drop behavior and undo support.
• Define how existing tasks enter a board without migration surprises.
• Decide whether one board is sufficient for v1.0 or multiple boards are required later.

Acceptance criteria

• The initial board model and migration strategy are documented before UI implementation.
• Existing tasks remain visible and editable without duplication.
• Cards can move between supported columns with undo.
• Task and Calendar views immediately reflect board changes.
• Search, accessibility, keyboard navigation, empty states, and populated states are covered.
• The feature is implemented in PyQt; Lite remains intentionally basic and CLI changes are limited to genuinely useful scripting operations.