fix: use viewport units for percentage map dimensions (fixes #2186)

[Kakarot35]

Fixes #2186

Problem

folium.Map(height='100%') collapses to nearly zero height when the map
is embedded inside a Django or Flask template. This happens because
height: 100% in CSS requires every ancestor element to have an explicit
height — which page templates typically don't have.

Similarly, folium.Map(height='1000px') can shrink in a non-maximized
browser window because there is no minimum floor set.

Changes

• Viewport units for percentages: when height or width is given as
  a percentage, emit vh/vw instead of %. Viewport units are always
  relative to the browser window size, not the ancestor chain, so they work
  correctly in both standalone and embedded (Django/Flask) contexts.

• Min constraints for pixel sizes: when height or width is given in
  pixels, also emit min-height/min-width so the map cannot shrink below
  its intended size in a smaller window.

• Remove dead CSS rule: the template was emitting
  #map { position:absolute; ... } which never matched anything — folium
  uses hashed IDs like map_d7a447c..., not map. Removed.

Before / After

/* Before — collapses in embedded templates */
#map_abc123 { height: 100%; }

/* After — always fills the viewport correctly */
#map_abc123 { height: 100vh; }

Tests

13 new tests added covering percentage → viewport units, pixel → min
constraints, dead rule removal, and full backwards compatibility.

[Kakarot35]

@hansthen The failing "Publish Docs" check was caused by a missing export in folium/plugins/init.py — now fixed and merged via #2230. This PR should be good to review now. Would appreciate your feedback!