chore: sync next with main (bring main-only fixes/features into next)

[dkastl]

Why

next has drifted from main: it is behind main by 9 commits (and ahead by 3). As a result next is missing changes that already landed on main, notably:

• enable to run vector format (GeoJSON) rendering (enable to run vector format(GeoJSON) #354)
• Ensure all assets are handled by webpack
• version bump to 6.0.2 in init.rb / package.json

This PR brings main into next so next ⊇ main again.

What

Merges main into next. The merge is conflict-free (git merge-tree reports no conflicts; 12 files auto-merge). next keeps its own 3 commits (the <hr>→<br> settings layout tweak and the test-assertion-warning fix).

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