Improve Explorer empty-state messages

[MuttleyD]

Summary

Improve the empty-state messages shown in the MIDAS Explorer when graph data is unavailable, empty, or fails to load.

Background

The Explorer is a key entry point for understanding MIDAS context and authority graphs. When graph data is missing or cannot be loaded, the current UI should make the state clear to the user.

A clearer empty state will help users distinguish between:

• no graph data being available
• graph data failing to load
• an unsupported or unavailable renderer state
• a valid but empty result set

This is a small frontend UX improvement intended to make the Explorer easier to understand and easier to troubleshoot.

Scope

Update the Explorer frontend empty-state handling so that users receive a clear message when graph content cannot be displayed.

The implementation should remain focused and should not require backend API changes unless there is already an obvious status or error value available to use.

Suggested approach

A suitable implementation could:

• identify the existing Explorer graph empty-state path
• replace vague or generic blank states with clearer user-facing messages
• ensure the message distinguishes between empty data and failed loading where that information is available
• keep the visual treatment consistent with the existing Explorer UI
• avoid introducing new frontend dependencies

Example message types could include:

• "No graph data is available for this view."
• "Graph data could not be loaded."
• "This renderer is not available for the selected view."

The exact wording can be adjusted to match the current MIDAS UI tone.

Acceptance criteria

• Explorer graph views show a clear empty-state message when no graph data is available.
• Explorer graph views show a clear error-state message when graph data fails to load, where failure information is available.
• The UI avoids presenting a blank graph area without explanation.
• The change is limited to frontend/static Explorer behaviour.
• The change does not alter graph layout behaviour.
• The change does not alter public APIs.
• The change does not introduce new dependencies.
• Existing tests continue to pass.
• go test ./... passes in CI.

Non-goals

• Do not redesign the Explorer.
• Do not change graph renderer selection behaviour.
• Do not change Cytoscape layout logic.
• Do not introduce a new design system.
• Do not add backend error contracts unless strictly necessary.
• Do not add browser automation unless already supported by the repository.

Contributor notes

This issue is intended to be a small, focused frontend improvement.

The goal is to make the Explorer clearer when there is nothing useful to render, not to redesign the graph experience.