add a hierarchical task view with hierarchy filters

Problem Statement

Kaneo supports parent/subtask relationships, but there is no dedicated view for understanding the full task hierarchy. Users must open individual task details to discover relationships, which makes it difficult to review or navigate work that is organized across multiple levels.

A flat task list also gives no quick way to distinguish independent tasks from tasks that participate in a hierarchy.

Proposed Solution

Add a hierarchical task view that renders parent tasks and their descendants as an expandable tree.

The view should include a filter with at least these modes:

• All tasks — show every task, including standalone tasks and hierarchical tasks.
• Tasks with hierarchy — show only tasks that participate in a parent/child hierarchy. This includes parent tasks and their descendants while excluding standalone tasks.

Suggested behavior:

• Indent each level to make parent/child relationships clear.
• Allow branches to be expanded and collapsed.
• Keep the relationship context visible when filtering, so a matching child is not shown without its ancestor path.
• Allow users to open a task directly from the hierarchy.
• Preserve the current filter selection while navigating within the project.
• Handle deeply nested tasks without making the view unusable on smaller screens.

Alternative Solutions

Users can continue navigating parent and subtask relationships from individual task detail pages. However, this only exposes one relationship at a time and does not provide an overview of the project's task structure.

Nested cards on the Kanban board, as discussed in #1170, could make immediate child tasks visible. That is useful for board workflows, but a dedicated hierarchy view is better suited to exploring multiple levels and filtering out standalone tasks.

Relevant Context

Kaneo already stores subtask relationships and displays them in task details. This request focuses on presenting those existing relationships in a project-wide structure rather than changing the underlying task model.

Suggested acceptance criteria:

• A project provides a hierarchical task view.
• Parent/child relationships are represented as an expandable tree.
• The All tasks filter includes both standalone and hierarchical tasks.
• The Tasks with hierarchy filter excludes standalone tasks and includes every task participating in a hierarchy.
• Descendants remain grouped under their correct ancestors.
• Selecting a task opens its existing task details.
• Empty, loading, and no-matching-results states are clearly represented.
• The view remains responsive for projects with many tasks.

Does this feature align with Kaneo's focus on simplicity?

Yes. It adds a focused way to inspect relationships that Kaneo already supports. The two filter modes keep the interaction small and predictable while helping users understand complex work without opening tasks one by one.

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Originally requested by @taiyoungjang on 2026-08-30. Original GitHub request #1679.