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)"
     />

frontend/src/components/NoteEditor.vue

@@ -264,7 +264,7 @@ async function act(fn: () => Promise<void>) {
       <div class="flex flex-col gap-2 p-4">
         <div v-if="liveNote.attachments.length" class="flex flex-wrap gap-2">
           <div v-for="att in liveNote.attachments" :key="att.id" class="group/att relative">
-            <img :src="att.url" alt="" class="h-24 w-24 rounded-lg object-cover" />
+            <img :src="att.url" alt="" loading="lazy" decoding="async" class="h-24 w-24 rounded-lg object-cover" />
             <button
               type="button"
               class="absolute right-1 top-1 rounded-full bg-black/50 px-1.5 text-white opacity-0 transition group-hover/att:opacity-100"