feat(web): ThemeSwitch icon segmented control + theme.* i18n (#59)

web/src/i18n/en.json

@@ -9,6 +9,7 @@
   "form": { "selectPlaceholder": "— select —", "create": "Create object", "save": "Save", "cancel": "Cancel", "visibility": "Visibility", "draft": "Draft", "internal": "Internal", "required": "This field is required", "rejected": "The server rejected the changes — check required and referenced fields", "fieldRejected": "The field \"{{field}}\" was rejected — check its value", "flexibleHeading": "Catalogue fields" },
   "actions": { "edit": "Edit", "delete": "Delete", "rename": "Rename", "save": "Save", "closeDetail": "Close detail", "confirmDelete": "Delete this object? This cannot be undone.", "confirmDeleteTerm": "Delete this term? This cannot be undone.", "confirmDeleteAuthority": "Delete this authority? This cannot be undone.", "confirmDeleteField": "Delete this field definition? This cannot be undone.", "confirmDeleteVocabulary": "Delete this vocabulary? This cannot be undone.", "inUse": "Can't delete — used by {{count}} object(s). Clear those fields first." },
   "labels": { "label": "Label", "externalUri": "External URI (optional)", "otherLanguages": "This entry also has labels in other languages, which are kept." },
+  "theme": { "light": "Light", "dark": "Dark", "system": "System" },
   "vocab": {
     "newVocabulary": "New vocabulary", "key": "Key",
     "create": "Create", "selectPrompt": "Select a vocabulary to manage its terms",