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/attachment/attachment-local/src/index.ts

@@ -6,10 +6,10 @@ import z from 'schemastery'
 import { AttachmentStore } from '@deepseek-ai/dsh-attachment'
 import type { ImageAttachmentLimits, ImageAttachmentRef, SaveImageAttachment, StoredImageAttachment } from '@deepseek-ai/dsh-attachment'
 import { resolveDshHome } from '@deepseek-ai/dsh-paths'
-import { readImageFile, saveImageFile } from './store.ts'
+import { readImageFile, saveImageFile, validateImageFile } from './store.ts'
 
 export { detectImage } from './image.ts'
-export { readImageFile, saveImageFile } from './store.ts'
+export { readImageFile, saveImageFile, validateImageFile } from './store.ts'
 export { AttachmentError } from '@deepseek-ai/dsh-attachment'
 export type { ImageAttachmentRef } from '@deepseek-ai/dsh-attachment'
 
@@ -62,6 +62,10 @@ export class LocalAttachmentStore extends AttachmentStore {
     })
   }
 
+  validateImage(input: SaveImageAttachment): void {
+    validateImageFile(input, this.imageLimits)
+  }
+
   async saveImage(input: SaveImageAttachment): Promise<ImageAttachmentRef> {
     return saveImageFile(this.root, input, this.imageLimits)
   }