refactor(street-level): simpler panel — native fullscreen, no FOLLOW button, no provider chips

EXPAND now uses the browser's Fullscreen API on the viewer element, which
stays in the panel; the hand-built modal (re-parenting to <body>, inert
siblings, focus trap, document-level Esc/Tab, Clean View/recording/cockpit
hide rules) is gone. The button is hidden where the API is missing.

The FOLLOW button and the PROVIDERS chip row are removed. The header pill is
the only layer switch and re-enables a provider switched off elsewhere; under
KEY REQUIRED the error line says which key to add. Chip colour support in
chipGroup, dead CSS and tests pinning the removed behaviour go with them.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

CHANGELOG.md

@@ -4,9 +4,9 @@
 
 - Street Level: a street-level imagery layer modelled on the iD editor's photo
   overlay, with Mapillary as its first provider (free client token, CC BY-SA 4.0
-  imagery). One right-rail panel holds a chip per provider, shared 360°/flat and
-  captured-since filters, and an embedded viewer with EXPAND, FIT/FILL and
-  FOLLOW (Google 3D only). Coverage lines and image cones are drawn from orbit
+  imagery). One right-rail panel holds an ON/OFF pill, shared 360°/flat and
+  captured-since filters, a legend, and an embedded viewer with full-screen
+  EXPAND and FIT/FILL. Coverage lines and image cones are drawn from orbit
   down to a single street; on Google 3D they sit on the bare earth, refined
   against the rendered mesh near the camera, so trees and buildings hide them.
   Share links carry the provider switches, the filters and the panel's collapsed