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-cs.json

@@ -499,10 +499,14 @@
     "full documentation": "úplná dokumentace",
     "items": "položky",
     "seconds": "sekund",
-    "theme-name-black": "Černý",
-    "theme-name-dark": "Tmavý",
-    "theme-name-default": "Výchozí",
-    "theme-name-light": "Světlý",
+    "theme": {
+        "name": {
+            "black": "Černý",
+            "dark": "Tmavý",
+            "default": "Výchozí",
+            "light": "Světlý"
+        }
+    },
     "{%device%} wants to share folder \"{%folder%}\".": "{{device}} chce sdílet složku „{{folder}}“.",
     "{%device%} wants to share folder \"{%folderlabel%}\" ({%folder%}).": "{{device}} chce sdílet složku „{{folderlabel}}“ ({{folder}}).",
     "{%reintroducer%} might reintroduce this device.": "{{reintroducer}} může toto zařízení znovu uvést."