Add git version control panel and LAN hosting support

- Git modal (⎇ Git button): commit all diagrams as JSON to diagrams/ folder,
  push to remote, view history, restore any diagram from any past commit
- Diagrams exported to diagrams/{id}_{name}.json on each commit so diffs are
  human-readable and individual diagrams can be restored independently
- backend/run.py: changed host to 0.0.0.0 for LAN access

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/run.py

@@ -1,4 +1,4 @@
 import uvicorn
 
 if __name__ == "__main__":
-    uvicorn.run("app.main:app", host="127.0.0.1", port=8000, reload=True)
+    uvicorn.run("app.main:app", host="0.0.0.0", port=8000, reload=True)