Feature request: Display card dependencies in the Gantt chart

Description

Deck supports dependencies between cards, but these dependencies are currently not visualized in the Gantt view.

For example, if:

Requirements → Design → Development → Testing

are configured as dependent cards in Deck, the Gantt chart should visually show these relationships.

Requested functionality

Display dependencies between cards in the Gantt chart using dependency lines/arrows.

For example:

Requirements █████████

Design █████████

Development ███████████

The exact visual representation can follow the conventions of the underlying Gantt component, but the relationship should be clearly visible.

Expected behaviour

If Card B is configured as dependent on Card A:

the Gantt chart should draw a visual connection from Card A to Card B;
the connection should remain correct when the cards are moved or their dates change;
dependencies should work regardless of whether the cards are in the same Deck stack or different stacks, where supported by Deck's dependency model;
removing the dependency in Deck should remove the corresponding dependency visualization from the Gantt chart.

The dependency visualization should use the existing Deck dependency relationship rather than requiring users to configure the dependency a second time specifically for the Gantt view.

Rationale

Dependencies are an important part of project planning. A timeline showing only the dates and durations of tasks does not communicate which activities depend on others.

Deck already supports card dependencies, so the Gantt view should visualize the relationships that already exist in the Deck data model.

Without dependency visualization, users have to switch between the Kanban/card view and the Gantt view to understand the relationship between tasks.

Environment

Observed with:

Nextcloud 34.0.2 (Spring 26)
Deck 1.18.3

Current behaviour

Card dependencies can be configured in Deck, but no dependency lines/arrows are displayed in the Gantt view.

Desired behaviour

Visualize existing Deck card dependencies directly in the Gantt chart.

Additional implementation note

The Gantt implementation uses Frappe Gantt, which supports task dependencies. The Deck Gantt implementation currently appears not to pass the existing Deck dependency information to the Gantt task objects.

This feature should therefore ideally integrate the existing Deck dependency model with the dependency functionality of the Gantt component.

Start by locating Deck's Gantt implementation and the construction of its Frappe Gantt task objects, then trace how existing card dependencies are represented in the Deck data model. Confirm how Frappe Gantt expects dependencies, and verify that dependency lines remain correct when dates or card positions change and disappear when a dependency is removed.