1.5-dev: Split mode state is incorrect in the menu when opening a new document

<!--See our full bug reporting guidelines at https://inkscape.org/contribute/report-bugs/ Writing a good bug report will ensure we'll be able to help efficiently. 🙂--> #### Summary: <!--Summarize the issue/suggestion concisely:--> Split mode state is incorrect when opening a new document in the current state #### Steps to reproduce: <!--Describe what you did (step-by-step) so we can reproduce:--> - open Inkscape with any document (empty document also works) - Switch Split mode from View \> Split Mode to "Split" - Open a new document from File \> New #### What happened? {width=900 height=563} The new document has the wrong split mode state. The document has "None" but menu shows otherwise. Resetting it back to None and the Split again will show the divider. #### What should have happened? New documents should have the "None" state which is correct. The menu should match the state. <!--Attach the sample file(s) highlighting the issue, if appropriate.--> #### Version info <!--Open 'Help > About' and click on the little bug icon in the bottom right corner that copies the debug information to your clipboard. For command line users, run 'inkscape --debug-info'. For Inkscape 1.0.2 and older, please manually add the Inkscape Version and Operating System Version. The Inkscape version is listed in the About dialog. For command line users, run 'inkscape -V' Paste the information in the empty space between the apostrophes below:--> ``` Inkscape 1.5-dev (a28888488f, 2026-03-20, custom) Compile (Run) GLib version: 2.84.1 GTK version: 4.18.6 (4.18.6) glibmm version: 2.83.1 gtkmm version: 4.18.0 libxml2 version: 2.12.10 libxslt version: 1.1.43 Cairo version: 1.18.2 (1.18.2) Pango version: 1.56.3 (1.56.3) HarfBuzz version: 10.4.0 (10.4.0) Poppler version: 25.02.0 OS version: Fedora Linux 42 (Workstation Edition) ``` <!--❤️ Thank you for filling in a new bug report, we appreciate the help! ❤️ Please be patient while we try to find the time to look into your issue. Remember that Inkscape is developed by volunteers in their spare time, we'll try our best to respond to all reports.--> <!--Please be careful when/after writing # for example in logs, code, or versions of linux - use inline code span - single backticks (`) before and after it, like this - `#1618` - use multi-line code block - triple backticks (```) to fence/enclose console logs - attach long logs as a text file.-->

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