fix: change password endpoint method from POST to PUT to match frontend

backend/main.py

@@ -1102,7 +1102,7 @@ def auth_status():
     """Health check endpoint."""
     return {"status": "ok", "service": "mitai-jinkendo", "version": "v9b"}
 
-@app.post("/api/auth/pin")
+@app.put("/api/auth/pin")
 def change_pin(req: dict, session: dict=Depends(require_auth)):
     """Change PIN/password for current user."""
     pid = session['profile_id']