Let OpenWeather Units fall through the settings cascade

The Units field carried a Default of "metric" and had no blank option, so
the global panel layer always set a value and locked organizations and
users out of changing it. Mirror the fix already applied to OpenSky's plan.

- Plugin descriptor: drop the "metric" Default and add a blank "Not set"
  option; runtime still falls back to metric when no layer sets it.
- Web App: add a "Not set" choice to the Units control and stop forcing
  metric back into the form on load.
- Test guards that the units field has no Default and offers a blank option.
- Rebuilt embedded frontend.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Web App/web/src/components/Settings.vue

@@ -741,6 +741,7 @@ const owHealthTs = ref(null)
 const OW_HEALTH_KEY = 'pv.openweather.health'
 
 const OW_UNITS_OPTS = [
+  { value: '', label: 'Not set' },
   { value: 'metric', label: 'Metric (°C)' },
   { value: 'imperial', label: 'Imperial (°F)' },
   { value: 'standard', label: 'Standard (K)' },
@@ -766,7 +767,6 @@ function owSourceLabel(k) {
 
 function fillOwForm() {
   for (const k of OW_KEYS) owForm[k] = owField(k).own || ''
-  if (!owForm.units) owForm.units = 'metric'
 }
 
 function applyOpenWeatherView(body) {