Fixes #7546 - Relation widget shows "No {{fields.name}}" when collapsed and search returns "No options"

[BiaDrab]

Summary

This PR addresses the relation widget bugs reported in issue #7546 and discovers an additional related issue during investigation.

Issues Found and Fixed

Issue #7546 (Primary)

1. Collapsed state showed placeholder - "No featured_entries" instead of selected entry title
2. Search functionality broken - Queries returned "No options" even with matching entries
3. New entries unselectable - Admin-created entries missing slug field required by relation widget

Additional Issue (Discovered)
4. Duplicate entry selection allowed - Same entry could be selected multiple times in the Top Posts list.

Root Causes

• List summary referenced non-existent '{{title}}' field instead of actual relation field '{{fields.featured_entries}}'
• Entries collection lacked slug field definition, causing newly created entries to be missing this required identifier
• Relation widget couldn't match entries without slug value
• No deduplication mechanism to prevent selecting the same entry twice

Solution:

Changed the list summary from '{{title}}' to '{{fields.featured_entries}}' to display the selected entry name
Added the slug field to the entries collection form so it's available in front matter
The relation widget now properly uses value_field: "slug" without extra braces

Testing

• Create entry via admin UI with title and slug
• Select entry in Top Posts dropdown - search works
• Collapsed state displays entry title (not "No featured_entries")
• Relation widget properly stores slug value
• Multiple entries can be added without duplicates
• Same entry cannot be selected twice in the same list

Configuration Changes

Homepage - Top Posts
summary: '{{fields.featured_entries}}' # Changed from '{{title}}'
# Entries collection - added slug field
- {label: "Slug", name: "slug", widget: "string", hint: "URL-friendly version (use hyphens, no spaces)"}