[sandesh-sp]

Part of #570. Stacked on #574 (feature/570-scale-bar). Merge that first, then retarget this PR to development.

Summary

• Bottom float zones clear the basemap chrome. In deck.gl overlay mode the MapLibre/Mapbox logo and attribution sit in the map's bottom corners, and bottom float panels (e.g. the scale bar in float-bottom-left) could overlap them. The modern layout now lifts bottom-left, bottom-right and bottom-center zones by how far that chrome reaches up (bottom-center uses the taller of the two sides).
• Engine contract: IMapEngine.getChromeInsets() returns, in CSS pixels, how far each bottom corner's chrome reaches up (bottomLeft, bottomRight) and in from its side (bottomLeftWidth, bottomRightWidth). DeckGLAdapter measures the basemap's bottom control corners with a ResizeObserver (they're filled after the map is created) and emits chromeinsetschange when the rounded values change. LeafletAdapter returns zeros, because its attribution control is removed at init.
• Bus API: map:getChromeInsets (request) and map:chromeInsetsChange (event). Documented in Event-Bus-API.md.
• Layout: UserInterfaceModern_ subscribes to the event, asks once on init in case the map reported first, and writes --ui-map-chrome-bottom-left/right on :root. A zone is lifted only for chrome beneath it: chrome beside the center region (under a side panel, in the overlay layout) is ignored, and any part a bottom panel already covers is subtracted, and the lift is recomputed when the center area resizes. Everything is cleaned up on teardown.

Test plan

• npx vitest run: 216 files, 3,648 tests pass (deckGLOverlayChrome.spec.js and userInterfaceModern.mapChrome.spec.js cover the measurement and the lift)
• deck.gl dashboard with a float-bottom-left panel (e.g. ScaleBar): the panel sits just above the MapLibre logo
• A float-bottom-right panel sits above the attribution, and stays clear when the attribution is expanded or collapsed
• With a bottom panel docked over the map, float panels aren't lifted more than necessary
• Overlay layout with a left panel open: the bottom-left float zone stays on the edge (the logo is under the panel)
• Leaflet engine: bottom float zones stay at the bottom edge

[sandesh-sp]

Replaced by #577: an admin-configured bottom offset for floating panels instead of measuring the basemap's logo and attribution at runtime.