v0.1.0

First working version.

• Render ```diagram blocks as images on every Kanboard Markdown surface.
• Edit an existing diagram from the rendered view or the editor preview, where
  Kanboard offers an edit action for the surrounding text.
• Insert a new diagram from the Markdown toolbar.
• Wiki.js-compatible payload: base64 of draw.io's xmlsvg export.
• frame-src added to Kanboard's Content-Security-Policy for the draw.io editor,
  merged into the existing rules rather than replacing them.
• DRAWIO_EMBED_URL points the editor at a self-hosted draw.io. A relative URL is
  treated as same-origin and adds no third-party origin to the policy.
• DRAWIO_MAX_PAYLOAD_SIZE refuses a diagram too large for the Markdown field
  before anything is written — MySQL stores descriptions and comments in a TEXT
  column, where an overflow truncates the whole field.