fix(starlark): blank app locations use the device location, not San Francisco

A Starlark (Tidbyt) app whose Location field is blank rendered at its
author's hard-coded DEFAULT_LOCATION -- usually San Francisco -- even with
the device city set under General settings. A user in Charlotte, NC got San
Francisco weather and radar with nothing in config.json to explain it.

src/device_location.py fills unset location fields at render time (display
plugin and the web standalone render): the device city is geocoded once via
Open-Meteo, preferring a match in the configured state/country, and cached
permanently. A saved location always wins; if the lookup fails the field is
dropped so the app uses its own default, and the failure is not retried for
30 minutes.

Also fixes the config form: clearing a location omitted the key, and the
save merges, so the old value could never be removed.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web_interface/templates/v3/partials/starlark_config.html

@@ -221,6 +221,7 @@
                                    data-starlark-location-key="timezone">
                         </div>
                     </div>
+                    <p class="text-xs text-gray-500 mt-1">Leave latitude and longitude blank to use this device's location (City / State / Country in General settings).</p>
                     {% if field_desc %}
                     <p class="text-xs text-gray-400 mt-1">{{ field_desc }}</p>
                     {% endif %}
@@ -421,9 +422,10 @@ function saveStarlarkConfig(appId) {
             var locKey = sub.getAttribute('data-starlark-location-key');
             if (sub.value) loc[locKey] = sub.value;
         });
-        if (Object.keys(loc).length > 0) {
-            config[fieldId] = JSON.stringify(loc);
-        }
+        // Blank still has to be sent: the save merges into the stored config,
+        // so leaving the key out kept the old location with no way to clear
+        // it back to the device's.
+        config[fieldId] = Object.keys(loc).length > 0 ? JSON.stringify(loc) : '';
     });
 
     fetch('/api/v3/starlark/apps/' + encodeURIComponent(appId) + '/config', {