feat(street-level): Google Street View as the second imagery provider

Google Street View registers beside Mapillary as provider `google`. It runs
on the Maps JavaScript API with the browser GOOGLE_MAPS_API_KEY, loaded on
first use (mapsLoader.js), and uses Google's own StreetViewPanorama as its
viewer adapter, emitting provider-neutral poses (address and month in the
caption, a link to Google Maps, steps along Google's arrows, FOLLOW on
Google 3D). Its chip starts off because every panorama is billed to the
key; the share option is `g`.

Google offers no coverage layer for other maps, so the provider draws
none: with it on, a click on bare ground at street zoom opens its nearest
Google-collected outdoor panorama (groundClick.js). A key Google refuses
reads KEY REJECTED and fails a pending open at once; a lookup Google never
answers fails after 10 s.

Core: providers may declare `groundClick` and `defaultOn`; poses may carry
`capturedAtPrecision` and `title`; the coverage line keeps every
provider's hint; opening one provider's image releases another's
prewarmed viewer; FIT/FILL hides for viewers without render modes. CSP
script-src admits https://maps.googleapis.com. The browser gate gains two
Street View steps (the keyed one skips as 'no Google key' in CI).

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

CHANGELOG.md

@@ -63,6 +63,13 @@
 
 ## [Unreleased]
 
+- Street Level: Google Street View is the second provider. 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 the panel, with its arrows, the address and month in the
+  caption, a link to Google Maps and FOLLOW on Google 3D. Share links carry
+  it as `g`.
+
 - Stdio servers from one install share a panel key so a page read from one
   process can make requests through another. Concurrent malformed-key repairs
   now select one winner; unavailable storage or a busy repair retains the