Intermittent live-node failure: Sent folder click sometimes leaves the view on Inbox #367

The live-node test "sent message stays in Sent (not Drafts) across reload (#286)" fails intermittently, about 1 in 86 runs, because the Sent folder click doesn't take effect: the view stays on Inbox.

What the evidence shows (gathered while finishing #366):

• The failure screenshot shows the Sent folder count at 1, so the row exists and no data is lost. The view shows Inbox, with the pointer still hovering over the Sent button.
• Ruled out:
  • a remount of UserInbox, which owns the folder state. A remount needs logged_id() to go None, and only deletion or logout do that; neither runs in this test.
  • a stray overlay or duplicate app root (checked in the failure HTML);
  • the folder button being replaced mid-click (its markup is static);
  • a page navigation, crash, dialog or console error.
• No reproduction since instrumenting: 45 of 45 runs passed at 842f427.
• Remaining hypotheses: the click event never reaches the app (for example, it arrives during a render, or before the listener is attached after a reload), or a reset path not yet identified.

Instrumentation now in place:

• release-visible log lines for UserInbox mount and unmount, app() branch changes (active id and identity count) and folder clicks;
• stable keys on the folder buttons;
• the test asserts Sent is selected immediately after the click (f8e65d3);
• on failure, the live-node suite keeps each page's console, navigations, crashes, a screenshot and the HTML in test-results/.

The next occurrence should fail at the click, with logs that tell a lost click from a reset. Fix it at the root then. No retries or timeout increases.

[AI-assisted - Claude]