Migrate Image Install/Delete to New GraphQL Queries

## Description

We plan to migrate the image **install (preload)** and **delete (unload)** workflows to use new GraphQL queries.

## Background

Image distribution to agents is currently handled via existing APIs/queries. A new set of GraphQL queries is being introduced, and once available, WebUI should use them as the primary mechanism for installing and deleting images on target agents.

## New Queries

- `preload_image(references: [String]!, target_agents: [String]!): PreloadImage`
- `unload_image(references: [String]!, target_agents: [String]!): UnloadImage`

## Scope

- Update the image **install** flow to call `preload_image` with:
- `references`: one or more image references
- `target_agents`: selected agent IDs
- Update the image **delete** flow to call `unload_image` with the same parameters.
- Ensure the UI behavior remains consistent (feature parity), including:
- validation (required fields, empty selections)
- success/error handling and user feedback
- loading/progress states (as supported by the API response)

## Acceptance Criteria

- Image install uses `preload_image` when the query is available.
- Image delete uses `unload_image` when the query is available.
- No functional regression compared to the current implementation.
- Proper error messages are shown when the operation fails for any image/agent.

## Notes

Implementation should be aligned with the finalized schema/response shape of `PreloadImage` and `UnloadImage` once the backend implementation is confirmed.

JIRA Issue: FR-1967