Add button and custom modal flow implemented

lib/create/createFlowUploadPurpose.ts

@@ -0,0 +1,8 @@
+/** Multipart field `purpose` for `POST /api/uploads` — keep in sync with server validation. */
+export const CREATE_FLOW_UPLOAD_PURPOSES = [
+  "communityAvatar",
+  "customMethodAttachment",
+] as const;
+
+export type CreateFlowUploadPurpose =
+  (typeof CREATE_FLOW_UPLOAD_PURPOSES)[number];