feat: add Excalidraw drawing canvas support for .excalidraw and .canvas files

[Adityakk9031]

Resolves #696

Description

This PR introduces native support for drawing canvases within the OpenKnowledge editor using the Excalidraw library (@excalidraw/excalidraw). Users can now create, open, and edit infinite canvas drawing boards by opening files with .excalidraw, .canvas, or .okdraw extensions.

What Changed

• Core Extension Support: Added .excalidraw, .canvas, and .okdraw file extensions to LINKABLE_ASSET_EXTENSIONS and implemented the isExcalidrawDocFile helper.
• Excalidraw Editor Component: Implemented ExcalidrawDocEditor.tsx in packages/app. It renders the <Excalidraw> canvas and serializes/deserializes canvas JSON snapshots directly to the underlying Y.Text('source') doc string with live CRDT synchronization.
• Activity Pool Integration: Configured EditorActivityPool.tsx to mount the ExcalidrawDocEditor component when an .excalidraw, .canvas, or .okdraw document is active.
• Dependencies & Changeset: Added @excalidraw/excalidraw dependency to packages/app and included .changeset/add-excalidraw-canvas-support.md.

Testing

• Created files named canvas.excalidraw, canvas.canvas, and canvas.okdraw and verified that the Excalidraw editor loaded correctly.
• Added shapes and drawings to the canvas, refreshed, and verified that all progress persisted into Y.Text('source').
• Verified that @inkeep/open-knowledge-core and @inkeep/open-knowledge-app compile and pass TypeScript typechecking cleanly.

[Adityakk9031]

@tim-inkeep have a look

[amikofalvy]

We are investigating how to add tldraw, but given it's more strict licensing we are preferring other options like Excalidraw and JSONCanvas.

[nexeck]

Excalidraw would be cool :-)

[Adityakk9031]

Thanks for the feedback @amikofalvy! That makes total sense regarding tldraw's licensing constraints.

I have updated this PR to switch from tldraw to Excalidraw (@excalidraw/excalidraw, MIT License).

Summary of updates in the latest commit (0b1581a6):

• Excalidraw Integration: Replaced tldraw with @excalidraw/excalidraw. Added drawing canvas support for .excalidraw, .canvas, and .okdraw files.
• CRDT Persistence: Added ExcalidrawDocEditor.tsx to serialize canvas JSON snapshots directly to Y.Text('source') with live bi-directional sync.
• Activity Pool & Core: Added isExcalidrawDocFile(), updated DocumentListEntrySchema, upload.ts, sidebar-drag.ts, and EditorActivityPool.tsx for lazy mounting.
• Conflicts & Changeset: Resolved all merge conflicts with main, passed typechecking, and updated the changeset file (.changeset/add-excalidraw-canvas-support.md).

Ready for another look when you have a chance!

[Adityakk9031]

@anubra266 please review all the changes