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

scripts/test-invariants.ts

@@ -96,6 +96,10 @@ class TestAttachmentStore extends AttachmentStore {
     mediaTypes: ['image/png'],
   }
 
+  validateImage(_input: SaveImageAttachment): void {
+    throw new Error('test invariant attachment store does not validate images')
+  }
+
   saveImage(_input: SaveImageAttachment): Promise<ImageAttachmentRef> {
     return Promise.reject(new Error('test invariant attachment store does not save images'))
   }