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/templates/layer-panels.html

@@ -113,7 +113,6 @@
             <div class="sl-providers" role="group" aria-label="Imagery providers">
               <span class="cctv-summary-label">PROVIDERS</span>
               <div id="sl-provider-chips" class="sl-chips"></div>
-              <button id="sl-nearest-btn" class="scene-btn sl-nearest-btn" type="button" hidden>OPEN STREET VIEW</button>
             </div>
             <div id="sl-error" class="sl-error" role="alert" hidden><span aria-hidden="true">⚠</span> <span id="sl-error-text"></span></div>
             <!-- A missing key gates the filters only: the chips above stay
@@ -134,7 +133,11 @@
               </div>
             </fieldset>
             <ul id="sl-legend" class="sl-legend" aria-label="Coverage colours"></ul>
-            <div id="sl-coverage-meta" class="sl-meta"></div>
+            <!-- The hint says how to open imagery; the button does it without a pointer. -->
+            <div class="sl-meta-row">
+              <div id="sl-coverage-meta" class="sl-meta"></div>
+              <button id="sl-nearest-btn" class="scene-btn sl-nearest-btn" type="button" hidden>OPEN STREET VIEW</button>
+            </div>
           </div>
         </div>
       </div>