feat(street-level): Street View polish — name, filters, legend, marker

- The chip reads GOOGLE STREET VIEW (and its caption link likewise).
- Providers declare the imagery filters they honour (`filters`, default
  pano + since). Street View declares none: every panorama is 360° and it
  draws no coverage to thin, so its lookups ignore the filters and the
  panel hides IMAGERY and SINCE while it is the only lit provider.
- The legend lists only providers that draw coverage lines, with
  Selected only beside one: Street View alone shows no legend.
- Providers may pick their on-globe marker (`marker`); Street View's is a
  Google-blue field-of-view cone from a white-ringed dot, in place of
  GEV's amber pin, and switches back for a Mapillary photo.

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

README.md

@@ -344,7 +344,7 @@ _The Space Missions layer replaying a Falcon 9 ascent — labeled `RECONSTRUCTED
 
 **Setup.** At [mapillary.com/dashboard/developers](https://www.mapillary.com/dashboard/developers), register an application with **Read** access and copy its **Client Token** (`MLY|…`). Paste it into **POWER UP → Mapillary**, or set `MAPILLARY_CLIENT_TOKEN` in `.env` and restart. Without it the panel reads **KEY REQUIRED**. **Google Street View** uses `GOOGLE_MAPS_API_KEY` with the **Maps JavaScript API** enabled for it in Google Cloud; its chip starts off, since every panorama is billed to the key.
 
-**Use.** Turn on **Street Level** under DATA LAYERS → Cameras, or with the pill in the STREET LEVEL panel header. Each provider has a chip in its own colour (Mapillary green). The **IMAGERY** (ALL / 360° / FLAT) and **SINCE** filters apply to every provider. Click a coverage line for its image cones and a cone to open the photo. On the **Google 3D** map (Google's terms keep Street View off other maps, where it does not appear), light **STREET VIEW** and click a street from below 3 km, or press **OPEN STREET VIEW** for the view centre, to open Google's panorama there; step along its arrows. Above the photo: **EXPAND**, **FIT / FILL** (Mapillary), and **FOLLOW** (the globe camera follows the photo's view; Google 3D map only). The caption links to the image on the provider's site, and the globe credits each provider that is on ("© Mapillary contributors, CC BY-SA 4.0", "Street View imagery © Google").
+**Use.** Turn on **Street Level** under DATA LAYERS → Cameras, or with the pill in the STREET LEVEL panel header. Each provider has a chip in its own colour (Mapillary green). The **IMAGERY** (ALL / 360° / FLAT) and **SINCE** filters apply to Mapillary; Street View panoramas are all 360°, so with only Street View lit the filters and legend are hidden. Click a coverage line for its image cones and a cone to open the photo. On the **Google 3D** map (Google's terms keep Street View off other maps, where it does not appear), light **GOOGLE STREET VIEW** and click a street from below 3 km, or press **OPEN STREET VIEW** for the view centre, to open Google's panorama there; step along its arrows. Above the photo: **EXPAND**, **FIT / FILL** (Mapillary), and **FOLLOW** (the globe camera follows the photo's view; Google 3D map only). The caption links to the image on the provider's site, and the globe credits each provider that is on ("© Mapillary contributors, CC BY-SA 4.0", "Street View imagery © Google").
 
 Drag the panel header to float the panel, and resize it from any edge. Double-click the header or collapse it to put it back in the rail. Share links carry the provider switches and filters (`0.m.0` Mapillary off, `0.r.google-1` Street View on, `0.p.p` panoramas only, `0.s.365` the last year). To add a provider, implement the contract in `src/layers/streetLevel/registry.js` and register it in `src/app/layers/streetLevel.js`.