Add Ambient station primary source toggle; swap location name to station

When configured, a new "Use as primary source" toggle appears in Settings
under Ambient Weather Station (defaults on). When on, the location header
shows the station name and "Ambient Weather Station" subtitle instead of the
city; the hero badge is hidden as redundant. Toggling off restores the city
name immediately and re-shows the hero badge.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/public/index.html

@@ -601,6 +601,13 @@
         <p class="settings-help">From ambientweather.net → Account → API Keys. Keys are stored locally in your browser.</p>
         <p id="ambient-status" class="settings-help hidden"></p>
       </div>
+      <div class="settings-row" id="ambient-use-row" style="display:none">
+        <span class="settings-label">Use as primary source</span>
+        <label class="switch-toggle">
+          <input id="ambient-use-toggle" type="checkbox" checked>
+          <span class="toggle-slider"></span>
+        </label>
+      </div>
 
       <div class="settings-section-title">Personal Weather Station</div>
       <div class="settings-row settings-row--block">