Add Group By (Priority, Category) to tasks page

[Copilot]

Tasks page lacked any grouping options beyond the default date-based sections. Adds a Group By dropdown alongside the existing filters to organize tasks by Priority or Category.

Changes

index.html

• Added #groupBySelect dropdown to the task filter bar with options: No Grouping, Group by Priority, Group by Category

src/app.ts

• Registered change listener on #groupBySelect → filterTasks()
• Extended filterTasks() with two new grouping modes:
  • Priority: High Priority → Medium Priority → Low Priority → Ungrouped
  • Category: alphabetical category groups → Ungrouped (tasks with no category)
  • Default (no grouping) retains existing Overdue → Due Today → Upcoming → No Due Date behavior

e2e/app.spec.ts

• Added 4 E2E tests covering dropdown visibility, priority grouping order, category grouping order, and revert-to-default behavior

Behavior

Tasks without a value for the selected group field are collected into an Ungrouped section at the end, consistent with the issue spec.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add Group By options to tasks</issue_title>
<issue_description>On the tasks page, add another dropdown to group tasks by

• Priority
• Category

When using those options, tasks that do not have a value will be at the end as ungrouped</issue_description>

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

• Fixes Add Group By options to tasks #32

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