fix: storage S3 stream conversion, API client FormData support, and attachment API

Fix S3 provider to use async generator for web stream conversion instead
of Readable.fromWeb, add postForm helper and attachment API methods to
the UI client, and add local disk storage provider tests.

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

ui/src/api/client.ts

@@ -1,8 +1,14 @@
 const BASE = "/api";
 
 async function request<T>(path: string, init?: RequestInit): Promise<T> {
+  const headers = new Headers(init?.headers ?? undefined);
+  const body = init?.body;
+  if (!(body instanceof FormData) && !headers.has("Content-Type")) {
+    headers.set("Content-Type", "application/json");
+  }
+
   const res = await fetch(`${BASE}${path}`, {
-    headers: { "Content-Type": "application/json" },
+    headers,
     ...init,
   });
   if (!res.ok) {
@@ -16,6 +22,8 @@ export const api = {
   get: <T>(path: string) => request<T>(path),
   post: <T>(path: string, body: unknown) =>
     request<T>(path, { method: "POST", body: JSON.stringify(body) }),
+  postForm: <T>(path: string, body: FormData) =>
+    request<T>(path, { method: "POST", body }),
   patch: <T>(path: string, body: unknown) =>
     request<T>(path, { method: "PATCH", body: JSON.stringify(body) }),
   delete: <T>(path: string) => request<T>(path, { method: "DELETE" }),