An expired session exits 3 with one line saying to run quak login (closes #164)

A 401 from the server that ends a command reaches run in src/cli-run.ts as
an ApiError; run prints one line saying to run "quak login" and exits 3.
quak backup meets it on its first refresh, before it touches any file. For
the commands that load the saved session, a missing or corrupt session file
keeps its message and also exits 3.

Judgement call: quak logout is unchanged; it handles its own errors and
deletes the session file whatever the server answers.
Judgement call: quak backup still prints its two progress lines before the
error line.

Model: opus-5-5

test/cli/run.test.ts

@@ -5,6 +5,7 @@
 import { PassThrough } from "node:stream";
 import { describe, it, expect } from "vitest";
 
+import { ApiError } from "../../src/api/client.js";
 import { run } from "../../src/cli-run.js";
 
 // A stream whose written text is kept in `text`; writes finish at once, so
@@ -55,4 +56,26 @@ describe("run", () => {
             stderr: "quak: offline\n",
         });
     });
+
+    it("on a 401 from the server says to run quak login, on one line, and exits 3", async () => {
+        const result = await runToExit(
+            Promise.reject(new ApiError("unauthorized", 401)),
+        );
+        expect(result).toEqual({
+            code: 3,
+            stdout: "",
+            stderr: `quak: the saved session is no longer valid; run "quak login"\n`,
+        });
+    });
+
+    it("prints another HTTP error as it is and exits 1", async () => {
+        const result = await runToExit(
+            Promise.reject(new ApiError("forbidden", 403)),
+        );
+        expect(result).toEqual({
+            code: 1,
+            stdout: "",
+            stderr: "quak: forbidden\n",
+        });
+    });
 });