fix(join-group): redirect unauthenticated users to login + docs update

- JoinGroupPage: redirect to login when user is not authenticated
- CLAUDE.md: add env details, path alias, and coding constraints

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

frontend/src/views/JoinGroupPage.vue

@@ -19,7 +19,7 @@ const error = ref('')
 
 const groupId = route.params.groupId as string
 
-onMounted(async () => {
+async function loadGroup() {
   try {
     group.value = await getGroup(groupId)
   } catch {
@@ -27,6 +27,14 @@ onMounted(async () => {
   } finally {
     loading.value = false
   }
+}
+
+onMounted(async () => {
+  if (!isAuthenticated()) {
+    router.replace({ name: 'Login', query: { redirect: route.fullPath } })
+    return
+  }
+  await loadGroup()
 })
 
 function goToLogin() {