chore: changelog entry for the phone layout

changelog.d/1325-phone-layout.md

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+- Add a phone layout for viewports 720px and narrower: the globe stays full
+  screen and a bottom tab bar (Layers, Cameras, Context, Search, More) opens one
+  panel sheet at a time, keeping attribution visible and scene STOP reachable.
+  Adapted from the phone tab-shell in #203 by blearymoth (samehkhamis, #1325).