fix(gui): address ds-review-bot findings on image attachments

- dsh web gains --provider/--model: a non-deepseek provider mounts the
  matching pi-ai catalog route (ambient credentials), making image input
  reachable from the shipped Web assembly; requires an explicit --model
- attachment-local syncs the publication directories after the hard-link
  publish so a reported durable reference survives a crash (POSIX; Windows
  relies on filesystem metadata journaling)
- the attachment seam gains storage-free validateImage; the host validates a
  complete multi-image prompt before persisting any member, so one malformed
  image cannot strand valid members as unreferenced objects
- startSession sends before navigating: a rejected first send keeps the empty
  state, its error strip, and the complete draft mounted
- the webserver rejects an undeclared-length body the moment it crosses the
  configured limit instead of draining a potentially endless stream to EOF

packages/host/runtime/src/api-proxy.ts

@@ -54,6 +54,16 @@ async function durablePromptContent(ctx: Context, content: readonly PromptConten
   if (totalBytes > limits.maxMessageImageBytes) {
     throw new AttachmentError('Prompt exceeds the configured aggregate image-byte limit.', 'IMAGES_TOO_LARGE')
   }
+  // Validate the complete batch before persisting any member: the store has no
+  // garbage collection, so one malformed image must not leave the batch's
+  // valid members as published objects no message event will ever reference.
+  for (const image of images) {
+    ctx.attachments.validateImage({
+      data: image.data,
+      mediaType: image.part.mediaType,
+      ...image.part.name === undefined ? {} : { name: image.part.name },
+    })
+  }
   return Promise.all(prepared.map(async (item): Promise<ContentBlock> => {
     if (!('data' in item)) return { type: 'text', text: item.text }
     const attachment = await ctx.attachments.saveImage({