fix(street-level): point a key-gated layer at Street View; button beside the hint

Review of #966: with a Google Maps key but no Mapillary token, switching
Street Level on read KEY REQUIRED / "Needs MAPILLARY_CLIENT_TOKEN" with
nothing about Street View, which stayed off (billing), so the layer looked
broken.

- While the switched-on providers lack their keys, the layer reports the
  switched-off ones that would draw with a key already set
  (keyAlternatives: Street View on Google 3D with a Google key). The
  panel's KEY REQUIRED tooltip, its coverage line and the layer row's
  tooltip then read "Needs MAPILLARY_CLIENT_TOKEN — add it in Provider
  Settings, or light STREET VIEW to use your Google Maps key"
  (keySetupWithAlternatives). Off Google 3D the plain KEY REQUIRED stays.
  Street View stays off by default.
- OPEN STREET VIEW moves beside the coverage hint: at the default panel
  width the PROVIDERS row has no room for it, so it wrapped onto a line of
  its own.
- The panel gate's corner drag names what lay under the pointer when it
  fails.

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

docs/CURRENT-STATE.md

@@ -3063,7 +3063,11 @@ The header pill is the layer switch. With one provider its chip is a layer
 switch too; with several, darkening the last lit chip turns the layer off. A
 chip without a usable key explains the key instead of lighting (or of
 switching the layer off as the last lit chip); it still darkens beside a lit
-one, and the other chips keep switching. The
+one, and the other chips keep switching. When the switched-on providers lack
+their keys but a switched-off one would draw with a key already set (no
+Mapillary token, Street View on Google 3D with a Google key), the layer
+reports it (`keyAlternatives`), and the panel's KEY REQUIRED, its coverage
+line and the layer row's tooltip say to light it. The
 viewer sits under the header with EXPAND, FIT/FILL, FOLLOW and close above the
 image. FOLLOW needs the Google 3D map stack (`attachMapStackController`) and
 stops when the stack changes. Camera moves go through the application's
@@ -3102,7 +3106,7 @@ for other maps, so it draws none: with it on, a click on the map itself
 Mapillary line under the pointer wins; no tool holding the pointer; camera
 below 3 km) opens its nearest Google-collected outdoor panorama within 50 m
 and leaves the camera where it is (`groundClick.js`). Higher up the hint
-says to zoom in. **OPEN STREET VIEW** in the PROVIDERS row (`#sl-nearest-btn`)
+says to zoom in. **OPEN STREET VIEW** beside that hint (`#sl-nearest-btn`)
 does the same at the view centre, so Street View needs no pointer; it is
 disabled until the camera is at street zoom. The viewer is Google's `StreetViewPanorama` in its own
 element in the viewer host, without Google's fullscreen, close and address