Improve the map legend

[jake-low]

What feature would you like to see added?

The "map controls" panel contains some rudimentary hints about what the colors and symbology of the map view represent:

But it would be good to more clearly describe what the colors and line styles mean. The current legend does not explain why some features are gray, or why some features use dashed lines while others use solid lines.

From https://github.com/OSMCha/osmcha-frontend/issues/788#issuecomment-2950835131

• turquoise green indicates elements that were created in the changeset
• red indicates elements that were deleted in the changeset
• yellow indicates elements whose tags were modified in the changeset
• gray indicates elements whose shapes were modified in the changeset. dark gray is used for the old shape and lighter gray for the new shape. If an element's tags and shape were both changed, yellow is used for the new shape (see above).
• purple indicates elements that were not changed but are being included on the map for context. This includes parent relations of ways that were modified, and children of relations that were modified.

There are also two different line styles used for ways:

• dashed lines indicate the old version of elements, from before the changeset
• solid lines indicate new versions of elements, after the changeset

Why would this be useful to you or other users?

It's not obvious to new users what the colors and styles mean (long-time users too, for that matter, since the colors and styles were changed recently). Explaining this helps make OSMCha more approachable.