Add button and custom modal flow implemented

lib/create/uploadToServer.ts

@@ -0,0 +1,93 @@
+import type { CreateFlowUploadPurpose } from "./createFlowUploadPurpose";
+
+export type UploadToServerResult = {
+  url: string;
+  id: string;
+  mimeType: string;
+  byteLength: number;
+};
+
+/**
+ * Authenticated multipart upload to `POST /api/uploads`.
+ * Caller must have a session cookie (same-origin fetch).
+ */
+export async function uploadCreateFlowFile(
+  file: File,
+  purpose: CreateFlowUploadPurpose,
+): Promise<UploadToServerResult> {
+  const formData = new FormData();
+  formData.append("purpose", purpose);
+  formData.append("file", file);
+
+  const res = await fetch("/api/uploads", {
+    method: "POST",
+    body: formData,
+    credentials: "same-origin",
+  });
+
+  let body: unknown;
+  try {
+    body = await res.json();
+  } catch {
+    body = {};
+  }
+
+  const errParts = (() => {
+    if (body && typeof body === "object" && "error" in body) {
+      const e = (body as {
+        error?: { message?: string; code?: string };
+      }).error;
+      if (!e) return { message: null as string | null, code: null as string | null };
+      return {
+        message: typeof e.message === "string" ? e.message : null,
+        code: typeof e.code === "string" ? e.code : null,
+      };
+    }
+    return { message: null, code: null };
+  })();
+
+  if (!res.ok) {
+    const fallback =
+      res.status === 413
+        ? "FILE_TOO_LARGE"
+        : res.status === 401
+          ? "UNAUTHORIZED"
+          : "UPLOAD_FAILED";
+    const code = errParts.code ?? errParts.message ?? fallback;
+    throw new UploadToServerError(res.status, code);
+  }
+
+  const data = body as {
+    url?: string;
+    id?: string;
+    mimeType?: string;
+    byteLength?: number;
+  };
+  if (
+    typeof data.url !== "string" ||
+    typeof data.id !== "string" ||
+    typeof data.mimeType !== "string" ||
+    typeof data.byteLength !== "number"
+  ) {
+    throw new UploadToServerError(res.status, "INVALID_RESPONSE");
+  }
+
+  return {
+    url: data.url,
+    id: data.id,
+    mimeType: data.mimeType,
+    byteLength: data.byteLength,
+  };
+}
+
+export class UploadToServerError extends Error {
+  readonly status: number;
+  readonly code: string;
+
+  constructor(status: number, code: string) {
+    super(code);
+    this.name = "UploadToServerError";
+    this.status = status;
+    this.code = code;
+  }
+}