Vendor a glyphicons-only stylesheet instead of the full Bootstrap 3 base

[adarshsm]

Closes #1820. Thanks @hansthen for the steer.

folium loads netdna's bootstrap-glyphicons.css for the Awesome Markers glyphicon font. Despite the name, that file is a full Bootstrap 3 base stylesheet — alongside the @font-face/.glyphicon rules it ships a normalize reset and global body rules (font, colour, background, margin). Loaded after Bootstrap 5, those body rules leak onto the host page and, for example, mis-position a user-added Bootstrap 5 modal (the original report).

This vendors folium/templates/glyphicons.css containing only the @font-face and .glyphicon/.glyphicon-* rules (fonts served from jsDelivr), referenced the same way as the existing leaflet.awesome.rotate.css. Icon(prefix="glyphicon") markers keep working exactly as before; the page-wide body/reset pollution is gone.

Not a customer-facing change — glyphicon markers are unaffected; the only difference is that folium stops overriding the host page's body styles. Added a regression test asserting the vendored file is used and the netdna URL is gone.

[adarshsm]

Pushed a follow-up that should fix the failing snapshot job.

What was wrong: the vendored stylesheet was linked from cdn.jsdelivr.net/gh/python-visualization/folium/folium/templates/glyphicons.css, which jsDelivr serves from the default branch — so it 404s until this PR is merged (curl → HTTP 404). In CI the .glyphicon rules never load, and for a released package the URL would also point at a mutable main. It's the same /gh pattern awesome_rotate_css uses, but that file already exists on main; a brand-new vendored file can't resolve that way pre-merge.

Fix (59a744c): inline the glyphicons-only rules into every Map as a <style> block, read once from the packaged folium/templates/glyphicons.css. No external stylesheet request, no chicken-and-egg, and it keeps the GH-1820 goal (no Bootstrap-3 body reset leaking onto the host page). The glyph font still loads from the stable bootstrap@3.3.7 npm files (HTTP 200). Locally: 23 test_folium.py tests pass, black/ruff clean, and glyphicon markers render (icon classes map to the inlined .glyphicon-* rules).

One thing that still needs doing: the 7 snapshot baselines legitimately change, because dropping the leaked Bootstrap body-reset alters rendering even for non-glyphicon maps (e.g. issue_1885). Those need regenerating from the CI Ubuntu/Chrome environment. The workflows on my latest push are sitting in action_required — if a maintainer approves the run, I'll grab the screenshots artifact, sanity-check the renders, and commit them as the updated baselines.