refactor(street-level): rename the Mapillary layer to street-level

Pure move: the layer id, panel id, CSS prefix, classes, exports and config
paths become provider-neutral (street-level, #street-level-panel, sl-);
the Mapillary-specific modules move under providers/mapillary/. Behaviour
is unchanged. The two sprite collections now register under distinct ids
instead of overwriting each other under 'mapillary'.

Co-Authored-By: Claude Fable 5.1 <[email protected]>

src/ui/layerPanel.js

@@ -32,7 +32,7 @@ const PANEL_GROUPS = [
   },
   {
     label: 'Cameras',
-    ids: ['cctv', 'recent-imagery', 'mapillary'],
+    ids: ['cctv', 'recent-imagery', 'street-level'],
   },
   {
     label: 'Infrastructure',
@@ -73,7 +73,7 @@ const PANEL_LABELS = {
   'ais-live-vessels': 'Live Vessels',
   bikeshare: 'Bike Share',
   cctv: 'Cameras',
-  mapillary: 'Street Level (Mapillary)',
+  'street-level': 'Street Level',
   'alpr-cameras': 'Mapped ALPR Cameras',
   'local-datacenters': 'Data Centers',
   'local-firms': 'Active Fires',