fix: saving Settings > Interface no longer turns the admin's default font into your own (#32497)

The Interface tab's Save sent fontFamily and imageCompressionSize every
time, read from the settings store, which already carries the admin's
Default Interface Settings for anything the user has not set. One Save
with nothing changed therefore stored the admin's default as a personal
value: the row switched from "Default" to "Custom", and a later change
to the admin default never reached that account.

Leave a value out of that Save while the user only inherits it, as
isDefaultSetting already decides for the "Default" label. Values the
user set keep saving as before.

src/lib/components/common/InterfaceSettings.svelte

@@ -246,8 +246,10 @@
 		}
 
 		saveSettings({
-			imageCompressionSize: imageCompressionSize,
-			fontFamily
+			...(isDefaultSetting('imageCompressionSize') ? {} : { imageCompressionSize }),
+			...(isDefaultSetting('fontFamily') && fontFamily === defaultFontFamily()
+				? {}
+				: { fontFamily })
 		});
 	};