feat(calendar): list due tasks from a task vdir beside the events

The sampler reads open VTODOs with a due date from
~/.local/share/tasks/<list>/*.ics (a vdirsyncer filesystem storage,
override with UDT_TASKS_DIR) and writes them to the cache as ISO
'task' lines. The card merges them into the week's rows by date, with
a checkbox before the title and an urgency colour: overdue critical,
due today warning, later label. Overdue tasks stay until done, so they
lead the table. Tasks never mark the month grid.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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