feat(notes): show loading and failed states for stacked notes

A note that couldn't load rendered a silent blank div. Add a calm
NoteState placeholder (spinner while loading, message + retry on
failure) and gate the read view on it; content still wins when present.

src/components/NoteState.vue

@@ -0,0 +1,48 @@
+<script lang="ts" setup>
+defineProps<{ status: "loading" | "failed" }>()
+defineEmits<{ retry: [] }>()
+</script>
+
+<template>
+  <div class="note-state">
+    <span
+      v-if="status === 'loading'"
+      class="loading loading-spinner loading-md"
+      aria-label="Loading note"
+    ></span>
+    <template v-else>
+      <p class="note-state-message">Couldn't load this note.</p>
+      <button type="button" class="note-state-retry" @click="$emit('retry')">
+        Retry
+      </button>
+    </template>
+  </div>
+</template>
+
+<style scoped lang="scss">
+.note-state {
+  display: flex;
+  flex: 1;
+  flex-direction: column;
+  align-items: center;
+  justify-content: center;
+  gap: 0.75rem;
+  padding: 2rem 1rem;
+  color: var(--color-base-content);
+  opacity: 0.55;
+  text-align: center;
+}
+
+.note-state-message {
+  margin: 0;
+}
+
+.note-state-retry {
+  padding: 0.25rem 0.85rem;
+  border: 1px solid currentColor;
+  border-radius: 0.375rem;
+  background: transparent;
+  color: inherit;
+  cursor: pointer;
+}
+</style>