Task filtering: add No Due Date group and preserve section grouping on category filter

[Copilot]

Tasks without a due date were incorrectly grouped under "Tasks Due Today", and selecting a category filter collapsed all groupings into a flat list.

Changes

• filtersActive no longer includes categoryFilter — category filtering now preserves the grouped view; only status/search filters use the flat list
• New "Overdue" section — incomplete tasks with a past due date are separated from "Due Today"
• "Due Today" scoped to today only — filter changed from !task.dueDate || task.dueDate <= today to task.dueDate && task.dueDate === today
• New "No Due Date" section — tasks without a due date get their own group instead of appearing under "Due Today"

Resulting section order: Overdue → Due Today → Upcoming Tasks → No Due Date

// Before: category filter killed grouping; no-due-date tasks appeared under "Due Today"
const filtersActive = categoryFilter || statusFilter || searchTerm;
const dueToday = filtered.filter(task => !task.dueDate || task.dueDate <= today);

// After
const filtersActive = statusFilter || searchTerm;
const overdue   = filtered.filter(task => !task.completed && task.dueDate && task.dueDate < today);
const dueToday  = filtered.filter(task => task.dueDate && task.dueDate === today);
const noDueDate = filtered.filter(task => !task.dueDate);

Original prompt

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This section details on the original issue you should resolve

<issue_title>Task filtering</issue_title>
<issue_description>When tasks don't have a due date they should be in a new group of no due date tasks

When filtering tasks by a category, there should still be due today, overdue, and no due date task</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Task filtering #30

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