Feature request: Add sorting options to the Gantt chart

Description

The Gantt view currently displays cards in the same order as they appear in their Deck stack. This means that the vertical order of tasks in the Gantt chart does not necessarily correspond to their chronological order.

For a Gantt chart, the chronological order of tasks is important for understanding the project timeline.

For example, cards may currently be displayed as:

Development
Testing
Requirements
Design
Deployment

while their actual timeline is:

Requirements
Design
Development
Testing
Deployment

Requested functionality

Add sorting options to the Gantt view, at minimum:

Start date (ascending) — recommended default for the Gantt view
Due date (ascending)
Deck/card order — retain the current behaviour

Ideally, the user should be able to select the sorting method from the Gantt view.

Expected behaviour

When Start date ascending is selected, cards with an earlier start date should appear above cards with a later start date.

Cards without a start date could either:

be displayed at the bottom, or
remain in their existing Deck order after the dated cards.

The sorting should only affect the presentation in the Gantt view; it should not change the actual card order in the Deck stack.

Rationale

A Gantt chart is primarily a timeline visualization. Chronological ordering makes it substantially easier to understand the sequence and timing of project activities.

The current behaviour is particularly confusing when the Kanban order and chronological order differ.

Environment

Observed with:

Nextcloud 34.0.2 (Spring 26)
Deck 1.18.3

Current behaviour

Cards are displayed according to their Deck stack/card order rather than according to their start or due date.

Desired behaviour

Provide an option to sort the Gantt rows chronologically, preferably by start date ascending.

Start by locating the Gantt view implementation and how it currently builds rows from Deck card order. Add selectable start-date, due-date, and card-order sorting without changing the Deck stack, then verify chronological ordering and the chosen handling of cards without dates in the Gantt view.