Add plus button to instantiation cells

[tibisabau]

Description

This change improves the discoverability of adding new assignment lines in instantiation cells by introducing a visible "+" button. Previously, users could only add new assignments by pressing "Return" from an existing assignment line, which was not intuitive. The implementation adds an IconButton with a Plus icon below the specializations list that, when clicked, adds a new assignment at the end and automatically focuses it.

The solution follows the existing UI patterns in the codebase by:

• Using the Plus icon from lucide-solid/icons/plus
• Using the IconButton component from catcolab-ui-components
• Providing a tooltip "Add new assignment" for discoverability
• Using consistent styling aligned with the left margin of the specializations list

Closes #891.

Checklist if Applicable

• The tests passed – pnpm --filter ./packages/frontend run test
• Linting passed – pnpm --filter ./packages/frontend run lint
• Documentation has been added
• CHANGELOG.md has been updated

[kasbah]

Closing, see #1040 (comment)