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]>

build/vite.js

@@ -20,8 +20,9 @@ import { panelBuildPlugin } from './panel.js';
 export const BROWSER_CSP = [
   "default-src 'self'",
   // The Bhote Koshi event's embedded media: the YouTube player API, the
-  // Facebook video SDK and X's post widget (src/data/bhoteKoshiEmbeddedMedia.js).
-  "script-src 'self' 'unsafe-eval' blob: https://www.youtube.com https://connect.facebook.net https://platform.twitter.com",
+  // Facebook video SDK and X's post widget (src/data/bhoteKoshiEmbeddedMedia.js);
+  // and the Maps JavaScript API that Street Level's Google Street View runs on.
+  "script-src 'self' 'unsafe-eval' blob: https://www.youtube.com https://connect.facebook.net https://platform.twitter.com https://maps.googleapis.com",
   "style-src 'self' 'unsafe-inline' https://fonts.googleapis.com",
   "font-src 'self' data: https://fonts.gstatic.com",
   "img-src 'self' data: blob: https:",