feat(street-level): OPEN STREET VIEW, and Street View only on Google 3D

- OPEN STREET VIEW in the PROVIDERS row opens Street View's nearest
  panorama at the ground under the middle of the screen, without moving
  the camera: Street View no longer needs a pointer. It is disabled (and
  says so) until the camera is at street zoom.
- Off the Google 3D map Street View does not appear at all: no chip,
  legend swatch, globe credit, layer source or button, and Google's script
  is not loaded (the viewer has no prewarm; the script loads on first use).
  The share option keeps the user's choice for when Google 3D returns.
- A ground click measures the camera height at the click rather than
  reading the cached flag.
- A chip without a usable key explains it instead of lighting; it darkens
  beside a lit chip, and the other chips keep switching, so a refused
  Street View key cannot lock Mapillary out.
- The gate checks that nothing of Street View shows off Google 3D even when
  a share link switches it on, presses OPEN STREET VIEW on Google 3D, and
  asserts that each chip it switches really switched.

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

CHANGELOG.md

@@ -66,7 +66,8 @@
 - Street Level: Google Street View is the second provider. On the Google 3D
   map (Google's terms keep it off other maps), light its chip (off by
   default: every panorama is billed to the Google Maps key, which needs the
-  Maps JavaScript API) and click a street to open Google's own panorama in
+  Maps JavaScript API) and click a street, or press OPEN STREET VIEW, to
+  open Google's own panorama in
   the panel, with its arrows, the address and month in the caption, a link
   to Google Maps and FOLLOW. Share links carry it as `g`.