Add description column to the overview report page

Problem/Motivation

The Entity Mesh overview page at /admin/reports/entity-mesh displays a table with three columns: Case, Count, and Details. While the count tells site maintainers how many issues exist, it provides no context about why each case type matters.

Without a description, editors and site owners must research the SEO, performance, or usability implications of broken links, HTTP targets, iframes, or malformed URIs on their own. This limits the report's value as an actionable audit tool.

Steps to reproduce

1. Enable the entity_mesh module.
2. Run an analysis to populate the data store.
3. Navigate to /admin/reports/entity-mesh.
4. Observe that the table shows case labels and counts but no explanation of the impact or urgency of each case type.

Proposed resolution

Add a Description column to the overview table, positioned between Count and Details. Each description is a concise one-line explanation

User interface changes

The overview table at /admin/reports/entity-mesh gains a Description column between Count and Details.