Add Drawio plugin

[youssefboutaleb]

Adds Drawio to the plugin directory.

Draw.io diagrams inside Kanboard Markdown. A fenced ```diagram block renders as the diagram it describes, with a View full size action for every reader and an Edit action wherever Kanboard already offers one, and the Markdown toolbar gains an Insert diagram button that opens the draw.io editor.

• Repository: https://github.com/youssefboutaleb/kanboard-plugin-drawio
• Release: https://github.com/youssefboutaleb/kanboard-plugin-drawio/releases/tag/v0.2.0
• License: MIT
• Compatible with: Kanboard >= 1.2.20 (requires PHP >= 8.1), verified against 1.2.53

Notes for reviewers:

• The download URL is a purpose-built release asset, not a GitHub source archive, so it extracts to Drawio/ as the installer expects. remote_install is true; the packaged archive was installed into a clean 1.2.53 and lists as Drawio 0.2.0 under Extensions.
• No database schema, no migrations, no routes, no controllers, and no Kanboard core files are modified. The Markdown field is the only storage, so disabling the plugin leaves existing content untouched.
• The plugin adds exactly one CSP directive, frame-src 'self' https://embed.diagrams.net, merged into the existing rules rather than replacing them. DRAWIO_EMBED_URL points it at a self-hosted draw.io instead, and a relative URL adds no third-party origin at all.
• Diagrams render as <img src="data:image/svg+xml;base64,…">, never as inline SVG, so a hostile payload cannot execute script.
• No third-party code is bundled. CI runs the test suite and php -l.
• Entry inserted case-insensitively between DiscordNotifier and duedate; plugins.json validates as strict JSON.