Fix MemoryMonitor blank on GPU-less deployments

gpu_memory_percent is serialized as JSON null (not omitted) when no
GPU is present, but the template checked `!== undefined`, which is
true for null too. Calling .toFixed() on that null then threw during
render, blanking the whole widget instead of just hiding the GPU row.
Only showed up on GPU-less hosts like a Lightsail instance, not on
this dev machine which has a GPU.

frontend/src/components/MemoryMonitor.vue

@@ -60,7 +60,7 @@ onUnmounted(() => {
       <span class="value">{{ stats.memory_percent.toFixed(1) }}% ({{ formatMemory(stats.memory_used_mb) }} / {{ formatMemory(stats.memory_total_mb) }})</span>
     </div>
 
-    <div class="stat" v-if="stats.gpu_memory_percent !== undefined">
+    <div class="stat" v-if="stats.gpu_memory_percent != null">
       <span class="label">GPU</span>
       <div class="bar">
         <div class="fill gpu" :style="{ width: stats.gpu_memory_percent + '%' }"></div>