[DresdenGman]

Implements the request in #1636 in an opt-in form.

LayerControl gains a use_local_storage option (default False). It is a folium-side option, in the same style as draggable, and is not passed through to the Leaflet control. When enabled, the rendered map includes a small script that stores the selected base layer and active overlays in localStorage, keyed by layer name and page path, restores them on load, and updates the stored state on baselayerchange, overlayadd, and overlayremove. With the option off — the default — the rendered output is unchanged.

I saw in the #1636 discussion that there was some hesitation about whether this belongs in folium itself, so I kept it strictly opt-in and self-contained. Happy to adjust the shape, or to close this if you would rather it live outside core.

Notes and limits:

• Layers are keyed by name, so two layers with the same name are not distinguished.
• The JavaScript is covered by rendering tests rather than a browser test, as my environment has no selenium stack. Test suite: 397 passed against a baseline of 395, with the same 4 pre-existing failures as the baseline (3 selenium import errors and 1 time_slider failure).