feat(chat): add vision input with drag-and-drop & paste images (M9.4)

Drag-and-drop or paste images into chat input. Thumbnail preview above
input, max 4 images per message. Images encoded as base64 and sent in
Claude vision format (multimodal content arrays). Image attach button
in chat input toolbar.

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

packages/app/src/components/chat/ChatMessage.vue

@@ -74,13 +74,23 @@
 
         <!-- Normal display -->
         <template v-else>
+          <!-- Attached images -->
+          <div v-if="message.images && message.images.length > 0" class="flex gap-2 flex-wrap mb-2">
+            <img
+              v-for="(img, i) in message.images"
+              :key="i"
+              :src="`data:${img.mediaType};base64,${img.data}`"
+              :alt="`Attached image ${i + 1}`"
+              class="rounded-lg max-w-[200px] max-h-[200px] object-cover border border-white/10"
+            />
+          </div>
           <div
             v-if="!isUser"
             class="chat-markdown text-sm leading-relaxed break-words text-white/90"
             v-html="renderedMarkdown"
           />
           <p
-            v-else
+            v-else-if="message.content"
             class="text-sm leading-relaxed whitespace-pre-wrap break-words text-white/90"
           >{{ displayText }}</p>
         </template>