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/tests/host-runtime.spec.ts

@@ -614,6 +614,31 @@ describe('sessions.prompt / cancel', () => {
     })
   })
 
+  it('publishes nothing when one member of a multi-image prompt is malformed', async () => {
+    const persistenceRoot = mkdtempSync(join(tmpdir(), 'dsh-batch-session-'))
+    const dshHome = mkdtempSync(join(tmpdir(), 'dsh-batch-home-'))
+    host = await startHost({
+      boot: { persistenceRoot, workspaceContext: false, dshHome, provider: 'scripted', model: 'test-model' },
+    })
+    host.ctx.llm.registerAdapter(['scripted'], new ScriptedAdapter([textResponse('unused')]))
+    const { sessionId } = expectOk(await host.api.sessions.create(request({})))
+    const response = await host.api.sessions.prompt(request({
+      sessionId,
+      mode: 'queue' as const,
+      content: [
+        { type: 'image' as const, mediaType: 'image/png' as const, data: PNG_BASE64 },
+        // Canonical base64, but the bytes are not a PNG: the whole batch must
+        // be validated before any member persists, or the valid image above
+        // would become a permanently unreferenced object (this store has no GC).
+        { type: 'image' as const, mediaType: 'image/png' as const, data: 'AQID' },
+      ],
+    }))
+    expect(response.result).toMatchObject({
+      ok: false, error: { details: { reason: 'INVALID_IMAGE' } },
+    })
+    expect(existsSync(join(dshHome, 'attachments'))).toBe(false)
+  })
+
   it('rejects images for an explicitly text-only model without creating a session event', async () => {
     const persistenceRoot = mkdtempSync(join(tmpdir(), 'dsh-text-session-'))
     const dshHome = mkdtempSync(join(tmpdir(), 'dsh-text-home-'))