gui: Use nested namespace for theme name translation keys (#9220)

Following up on #9192, this makes use of the new mechanism for the theme
names.

The dummy string added for testing is removed again here. All
translations are updated to the new nested syntax, except Chinese
(zh-HK) where the string weren't actually translated.

gui/default/assets/lang/lang-fr.json

@@ -538,10 +538,14 @@
     "modified": "modifié",
     "permit": "partager tous les attributs",
     "seconds": "secondes",
-    "theme-name-black": "Noir",
-    "theme-name-dark": "Sombre",
-    "theme-name-default": "Par défaut (système)",
-    "theme-name-light": "Clair",
+    "theme": {
+        "name": {
+            "black": "Noir",
+            "dark": "Sombre",
+            "default": "Par défaut (système)",
+            "light": "Clair"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} vous invite au partage \"{{folder}}\".",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} vous invite au partage \"{{folderlabel}}\" ({{folder}}).",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} pourrait ré-enrôler cet appareil."