fix(down): conditionally emit progress events for dangling images

[hardikkaurani]

Summary

This PR fixes a UI bug where docker compose down --rmi local hangs on a 'Removing' progress event when there are no dangling images to remove. It makes the progress event conditional.

Problem

The image pruner unconditionally emits a 'Removing' progress event before verifying if any dangling images actually exist, causing the CLI output to hang on an incomplete event.

Root Cause

The removeDanglingImages function triggers the api.Working event before filtering the images, failing to close the event if the list of targets is empty.

Changes

• Pre-calculates the toRemove list in removeDanglingImages.
• Accepts an onRemoving callback to conditionally emit the progress event only if removals begin.

Tests

Regression coverage is included for the affected behavior.

Why This Is Not a Duplicate

This PR addresses a different issue from #14251.

• Different scope: This PR patches executable Go code in the image pruner, while docs: remove broken CONTRIBUTING.md link #14251 is a documentation-only fix for a Markdown file.
• Different files: pkg/compose/image_pruner.go vs CONTRIBUTING.md.
• No shared runtime behavior.

Therefore, the two PRs address independent problems and this change does not supersede or duplicate #14251.

Scope

This change is limited to dangling image removal in docker compose down and watch and does not alter container lifecycle management.

Original Template Items

Fixes #14219.

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