prd: Relation side panel improvements

Problem

1. When adding a new relations, it gets added at the bottom of the list, requiring scrolling to label it.
2. There is no easy way to hide/show all relations at once.

Proof of Concept (PoC) Notes

A working branch implementing the relations list ordering and visibility toggle can be found at:
https://github.com/Precious-Macaulay/label-studio

Key highlights:

• Added ordering dropdown to toolbar with creation time options
• Added visibility toggle icon next to "Relations" label
• State is persisted in local storage

User Stories

Relations list ordering

As a user, I want to be able to order the relations list by creation time (ascending or descending) so that I can easily find and label new relations.

Relations visibility toggle

As a user, I want a toggle to hide/show all relations at once so that I can quickly analyze connections between entities.

Acceptance Criteria

Relations side panel list ordering

• Adds options to order relations list by creation time (ascending and descending)
• Unit tests for ordering functionality

Relations visibility toggle

• Adds a toggle icon next to "Relations" label
• Toggle hides all relationships when one or more are visible
• Toggle shows all relationships when all are hidden
• Persists visibility state between sessions
• Unit tests for visibility functionality