Add FileInteractionCore plugin

[youssefboutaleb]

Adds FileInteractionCore to the official Kanboard plugin directory.

Preview and edit task attachments without leaving Kanboard. Word, PowerPoint, Excel, PDF, CSV, Markdown, HTML, JSON and source code render in a modal; text, code, CSV and spreadsheet attachments can be edited in place.

• Repository: https://github.com/youssefboutaleb/kanboard-file-attachment-interaction
• Release: https://github.com/youssefboutaleb/kanboard-file-attachment-interaction/releases/tag/v1.1.1
• License: MIT
• Compatible with: Kanboard >= 1.2.23 (requires PHP >= 8.1)

Notes for reviewers:

• The download URL is a purpose-built release asset, not a GitHub source archive, so it extracts to FileInteractionCore/ as the installer expects. remote_install is true.
• No database schema, no migrations, no configuration, and no Kanboard core files are modified.
• Images, audio and video are deliberately excluded — core already renders those, and excluding them keeps active content such as .svg out of every preview path.
• CI runs PHPUnit (770 tests) and PHPStan level 8 on PHP 8.1–8.4.
• Entry inserted alphabetically between EssentialTheme and FontSwitcher; plugins.json validates as strict JSON.