feat(pool): an admin's switch moves a user's Claude login at once

PUT /api/accounts/pins (and agenthub accounts pin) now writes a pinned
Claude login into the user's home right away (claude-pool.switchHome):
every home's newer pair is copied back first, the member's own login is
never written over, and no pane is restarted — running sessions take the
new pair at their next credentials read. A live home stays on the login it
holds, so unpinning does not flip it back. The Users page calls it
"Switch a user's account" and says what happened.

server/__tests__/server-mode-account-pool.test.js

@@ -162,6 +162,7 @@ describe("pinning a user to one account", () => {
     const r = await pin(boss, { userId: miaId, account: "team-z" });
     assert.equal(r.status, 200);
     assert.deepEqual(r.body.pin, { userId: miaId, username: "mia", account: "team-z" });
+    assert.equal(r.body.switched, "next-session", "a setup-token goes in at the next spawn");
     assert.equal(claudePool.pickAccount(null, miaId).name, "team-z");
     assert.equal(claudePool.pickAccount(null, null).name, "team-1", "nobody else moves");
     const audit = db.prepare("SELECT owner_id, path FROM audit_log ORDER BY id DESC LIMIT 1").get();