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]>

src/ui/styles/street-level.css

@@ -183,10 +183,12 @@ button.sl-status:focus-visible {
   border-color: rgba(255, 179, 0, 0.55);
 }
 
-/* On, but not usable on this map stack (Street View off Google 3D). */
-.sl-chips .data-toggle-chip.chip-warn {
-  opacity: 0.7;
-  border-style: dashed;
+.sl-nearest-btn {
+  margin-left: auto;
+}
+
+.sl-nearest-btn[hidden] {
+  display: none;
 }
 
 /* ── Errors ──────────────────────────────────────────────────────────────── */