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

frontend/src/views/LifeView.vue

@@ -218,10 +218,14 @@ function resetForm() {
 function payload() {
   return {
     body: body.value.trim(),
-    tagIds: selectedTagIds.value.map((tagId) => Number(tagId)).filter((tagId) => Number.isInteger(tagId) && tagId > 0)
+    tagIds: selectedLifeTagIds()
   };
 }
 
+function selectedLifeTagIds() {
+  return selectedTagIds.value.map((tagId) => Number(tagId)).filter((tagId) => Number.isInteger(tagId) && tagId > 0);
+}
+
 function submitSearch() {
   const nextSearch = searchDraft.value.trim();
   if (nextSearch === submittedSearch.value && !loadError.value) {
@@ -281,6 +285,12 @@ async function submitPost() {
     return;
   }
 
+  if (selectedLifeTagIds().length === 0) {
+    formError.value = t('pages.life.tagRequired');
+    document.getElementById('life-post-tags')?.focus();
+    return;
+  }
+
   busy.value = true;
   formError.value = '';