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]>

src/ui/streetLevelControls.test.mjs

@@ -305,7 +305,6 @@ function panelDom() {
   const settings = add(main, 'div', { classes: ['sl-settings'] });
   for (const id of ['sl-provider-chips', 'sl-error', 'sl-error-text'])
     add(settings, 'div', { id });
-  add(settings, 'button', { id: 'sl-nearest-btn' });
   // A missing key gates the filters only; the chips stay outside the gate.
   const controls = add(settings, 'fieldset', { id: 'sl-controls' });
   for (const pano of ['all', 'pano', 'flat'])
@@ -313,7 +312,9 @@ function panelDom() {
   add(controls, 'input', { id: 'sl-since' });
   add(controls, 'output', { id: 'sl-since-label' });
   add(settings, 'ul', { id: 'sl-legend' });
-  add(settings, 'div', { id: 'sl-coverage-meta' });
+  const metaRow = add(settings, 'div', { classes: ['sl-meta-row'] });
+  add(metaRow, 'div', { id: 'sl-coverage-meta' });
+  add(metaRow, 'button', { id: 'sl-nearest-btn' });
   // The globe's canvas is focusable too (tabindex=0).
   const globe = add(
     add(document.body, 'div', { id: 'cesiumContainer' }),