Show dependencies between rows

## Goal

For task-flavored collections, let one row block another. Show blocked-by and blocking on the row detail view, and put the relationships somewhere a timeline view can use them.

## Scope

* A dependency relationship between two rows in the same collection: A blocks B, B is blocked by A.
* Add and remove dependencies from the row detail view.
* Surface blocked-by and blocking lists on the row detail.
* Expose dependencies to the timeline layout for arrow rendering.

## Out of scope

* Auto-rescheduling based on dependencies.
* Cross-collection dependencies.
* Critical-path computation.

## Acceptance criteria

* You can mark row A as blocking row B; the inverse appears on B without manual entry.
* Removing a dependency cleans up both sides.
* Timeline layout draws arrows between blocker and blocked.
* Cycles (A blocks B blocks A) are caught and surfaced as a warning, not silently saved.

Start by locating the row detail view and timeline layout entry points, then trace how relationships between rows are represented within a collection. Done means dependencies can be added and removed with inverse links maintained, blocked-by and blocking lists are shown, timeline arrows render, and cycles produce a warning instead of being saved.

Written by the indexing model from the issue text.