feat(life): require at least one tag for life posts

Update design spec to mandate tag selection
Add frontend and backend validation for tag requirement
Add localization strings for tag required errors

backend/src/queries.ts

@@ -2089,10 +2089,14 @@ function cleanLifePostPayload(payload: Record<string, unknown>): LifePostPayload
   if (body.length > 2000) {
     throw validationError('Post is too long');
   }
+  const tagIds = cleanIds(payload.tagIds);
+  if (tagIds.length === 0) {
+    throw validationError('server.validation.lifeTagRequired');
+  }
 
   return {
     body,
-    tagIds: cleanIds(payload.tagIds)
+    tagIds
   };
 }