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/app/main.py

@@ -7,7 +7,7 @@ from fastapi.staticfiles import StaticFiles
 from sqlalchemy import text, inspect as sa_inspect
 
 from .database import Base, engine, SessionLocal
-from .routers import bundles, connectors, diagrams, devices, export, octopart, views, wires
+from .routers import bundles, connectors, diagrams, devices, export, git_mgr, octopart, views, wires
 
 Base.metadata.create_all(bind=engine)
 
@@ -70,6 +70,7 @@ app.include_router(views.router,     prefix="/api/views")
 app.include_router(octopart.router,  prefix="/api/octopart")
 app.include_router(export.router,    prefix="/api/export")
 app.include_router(connectors.router, prefix="/api/connectors")
+app.include_router(git_mgr.router,   prefix="/api/git")
 
 FRONTEND_DIR = os.path.normpath(
     os.path.join(os.path.dirname(__file__), "..", "..", "frontend")