feat(network): thumbnail generation on save, shown on list page

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/networkDiagrams.ts

@@ -51,6 +51,10 @@ export const networkDiagramsApi = {
     return response.data
   },
 
+  async uploadThumbnail(id: string, dataUrl: string): Promise<void> {
+    await apiClient.post(`/network-diagrams/${id}/thumbnail`, { data_url: dataUrl })
+  },
+
   async aiGenerate(data: AIGenerateRequest): Promise<AIGenerateResponse> {
     const response = await apiClient.post<AIGenerateResponse>('/network-diagrams/ai-generate', data)
     return response.data