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/llm/llm-pi-ai/tests/adapter.spec.ts

@@ -226,6 +226,10 @@ describe('PiAiAdapter provider routing', () => {
         mediaTypes: ['image/png'],
       }
 
+      validateImage(_input: SaveImageAttachment): void {
+        throw new Error('not used')
+      }
+
       saveImage(_input: SaveImageAttachment): Promise<ImageAttachmentRef> {
         return Promise.reject(new Error('not used'))
       }

packages/llm/llm-pi-ai/tests/provider-apis.e2e.ts

@@ -74,6 +74,10 @@ async function harness(image?: StoredImageAttachment): Promise<Context> {
         mediaTypes: [fixture.ref.mediaType],
       }
 
+      validateImage(_input: SaveImageAttachment): void {
+        throw new Error('e2e attachment fixture is read-only')
+      }
+
       saveImage(_input: SaveImageAttachment): Promise<ImageAttachmentRef> {
         return Promise.reject(new Error('e2e attachment fixture is read-only'))
       }