fix(web): LabelEditor preserves other-language labels on edit (#55)

Editing a term/authority/field that already had labels in other languages
silently replaced the whole multilingual set with one default-language entry.
onChange now keeps non-default-language entries; the editor shows only the
default-language label (no longer falling back to an other-language one, which
made the clear/edit path write the wrong language) and surfaces a hint when
other-language labels exist on the record.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/i18n/en.json

@@ -8,7 +8,7 @@
   "visibility": { "draft": "Draft", "internal": "Internal", "public": "Public" },
   "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)" },
+  "labels": { "label": "Label", "externalUri": "External URI (optional)", "otherLanguages": "This entry also has labels in other languages, which are kept." },
   "vocab": {
     "newVocabulary": "New vocabulary", "key": "Key",
     "create": "Create", "selectPrompt": "Select a vocabulary to manage its terms",