m4: perf pass — lazy-load note images

Light, audit-first perf pass (task 1870):
- lazy-load + async-decode the card thumbnail and editor attachment
  images so an image-heavy board / large note doesn't eagerly fetch
  and decode every off-screen image up front

Audit found the rest already in good shape and left as-is: all routes
are code-split via dynamic import(); the titles index (palette,
[[-autocomplete, wiki-link resolution) is cached in its store and only
fetched once; board refetches are scoped to real view/label changes with
no double-load on mount.

(Server-side thumbnail generation — cards load full-res uploads — is a
larger, separate feature, noted for later, out of scope for a light pass.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/NoteCard.vue

@@ -142,6 +142,8 @@ onBeforeUnmount(() => document.removeEventListener("mousedown", onDocMousedown))
       v-if="note.attachments.length"
       :src="note.attachments[0].url"
       alt=""
+      loading="lazy"
+      decoding="async"
       class="mb-2 max-h-48 w-full cursor-pointer rounded-lg object-cover"
       @click="emit('open', note)"
     />