swarm-ui: forms share one submit flow, result line and field column

useSubmit holds the idle, sending, done and error state and the request
flow that six forms each wrote out; FormResult shows the success line and
the problem panel; FormStack is the column the link, claude-login,
inference and limits forms each styled by hand. The create-agent, link
account, backend-env and agent-input forms use the submit flow; the link,
claude-login, inference and limits forms use FormStack.

frontend/packages/swarm-ui/test/submit.test.mjs

@@ -0,0 +1,58 @@
+// The request flow every form shares: a refusal becomes a problem, a failure
+// to send becomes a problem, a success parses its reply.
+import assert from "node:assert/strict";
+import { test } from "node:test";
+import { submitOutcome } from "../src/ui/submit/submit.ts";
+
+const problem = (status, body) =>
+  new Response(JSON.stringify(body), {
+    status,
+    headers: { "content-type": "application/problem+json" },
+  });
+
+test("an ok reply is done, with nothing parsed by default", async () => {
+  const out = await submitOutcome(
+    async () => new Response(null, { status: 204 }),
+  );
+  assert.deepEqual(out, { status: "done", value: undefined });
+});
+
+test("an ok reply is parsed into the done value", async () => {
+  const out = await submitOutcome(
+    async () => new Response(JSON.stringify({ node_id: 7 })),
+    { parse: async (r) => (await r.json()).node_id },
+  );
+  assert.deepEqual(out, { status: "done", value: 7 });
+});
+
+test("a refusal carries the server's problem", async () => {
+  const out = await submitOutcome(async () =>
+    problem(409, { title: "conflict", detail: "already linked" }),
+  );
+  assert.equal(out.status, "error");
+  assert.equal(out.problem.detail, "already linked");
+});
+
+test("refine can reword a refusal by status", async () => {
+  const out = await submitOutcome(async () => problem(404, {}), {
+    refine: (r, p) =>
+      r.status === 404 && !p.detail ? { ...p, detail: "no such agent" } : p,
+  });
+  assert.equal(out.status, "error");
+  assert.equal(out.problem.detail, "no such agent");
+});
+
+test("a request that throws is an error with the message", async () => {
+  const out = await submitOutcome(async () => {
+    throw new Error("offline");
+  });
+  assert.equal(out.status, "error");
+  assert.match(out.problem.detail, /offline/);
+});
+
+test("a parse that throws is an error, not a done", async () => {
+  const out = await submitOutcome(async () => new Response("not json"), {
+    parse: (r) => r.json(),
+  });
+  assert.equal(out.status, "error");
+});