Use File system API to manage local folders of diagrams

[gissehel]

Use File system access API to manager local folder diagrams.

• The way logseq demo site works might be interesting (bind one or several folders, and then manage data in it, while keeping a static site on the server side)

• The current usage should still works (no binding => the current GUI should be almost identical)

• With folder binded, there should have a way - an easy way - to select a diagram and update it. Should update both the source file, and the rendered file. Kroki should be able to render png/jpeg(/else?) even for formats that kroki doesn't provide

• When binded to a folder, kroki should be able to generate diagrams which sources are too big to fit in a url (which can't work right now by design)

• Before coding anything, the new GUI should be well designed