fix: polish UX error handling across views (FINAL-01)

- AppDetails: replace alert() with dismissible toast, add error feedback
  for start/stop/restart/uninstall actions
- GoalDetail: add error toast for install failures instead of silent catch
- Apps: add loading skeleton when WebSocket data hasn't arrived yet
- Add appDetails.noLaunchUrl i18n key

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

neode-ui/src/views/GoalDetail.vue

@@ -137,6 +137,16 @@
         </RouterLink>
       </div>
     </template>
+
+    <!-- Action error toast -->
+    <Transition name="fade">
+      <div v-if="actionError" class="fixed bottom-20 left-1/2 -translate-x-1/2 z-50 max-w-md w-full px-4" role="alert" aria-live="assertive">
+        <div class="bg-red-500/20 border border-red-500/40 backdrop-blur-sm rounded-lg px-4 py-3 text-red-200 text-sm flex items-center justify-between gap-3">
+          <span>{{ actionError }}</span>
+          <button @click="actionError = ''" class="text-red-300 hover:text-white shrink-0">&times;</button>
+        </div>
+      </div>
+    </Transition>
   </div>
 </template>
 
@@ -159,6 +169,14 @@ const goalId = computed(() => route.params.goalId as string)
 const goal = computed(() => getGoalById(goalId.value))
 
 const isInstalling = ref(false)
+const actionError = ref('')
+let errorTimer: ReturnType<typeof setTimeout> | undefined
+
+function showActionError(msg: string) {
+  actionError.value = msg
+  if (errorTimer) clearTimeout(errorTimer)
+  errorTimer = setTimeout(() => { actionError.value = '' }, 5000)
+}
 
 const overallStatus = computed(() => goalStore.getGoalStatus(goalId.value))
 
@@ -252,7 +270,7 @@ async function installApp(step: GoalStep) {
     await appStore.installPackage(step.appId, '', 'latest')
     goalStore.completeStep(goalId.value, step.id)
   } catch (err) {
-    if (import.meta.env.DEV) console.error('[GoalDetail] Install failed:', err)
+    showActionError(`Install failed: ${err instanceof Error ? err.message : 'Unknown error'}`)
   } finally {
     isInstalling.value = false
   }
@@ -278,3 +296,14 @@ function goBack() {
   router.push('/dashboard')
 }
 </script>
+
+<style scoped>
+.fade-enter-active,
+.fade-leave-active {
+  transition: opacity 0.3s ease;
+}
+.fade-enter-from,
+.fade-leave-to {
+  opacity: 0;
+}
+</style>