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>

API Server/internal/plugins/builtin/openweather/openweather_test.go

@@ -38,6 +38,23 @@ func TestDescriptor(t *testing.T) {
 		if f.Required {
 			t.Errorf("config field %q must not be Required", f.Key)
 		}
+		// Units must be blank-able so the value can fall through the settings
+		// cascade: a blank "" option and no Default, otherwise the global layer
+		// always sets it and locks organizations/users out.
+		if f.Key == "units" {
+			if f.Default != "" {
+				t.Errorf("units field must not carry a Default (got %q) — it would lock lower cascade layers", f.Default)
+			}
+			hasBlank := false
+			for _, o := range f.Options {
+				if o.Value == "" {
+					hasBlank = true
+				}
+			}
+			if !hasBlank {
+				t.Error("units field must offer a blank \"\" (Not set) option so it can fall through the cascade")
+			}
+		}
 	}
 }