Channels scroll listener throws after leaving the page; two mobile tap targets below 48 px #314

Two small pre-existing UI defects found in a browser check of v0.2.2-rc1 on staging. Neither is a regression; both are present in v0.2.1.

1. Console error from the channel message scroll listener

When you leave the Channels page, the console logs:

Uncaught TypeError: Cannot read properties of null (reading 'classList')
    at HTMLDivElement.<anonymous> (channels.js:1668)

public/channels.js (~1664-1669) registers a scroll listener on #chMessages. The listener calls document.getElementById('chScrollBtn').classList.toggle(...). A scroll event can still fire after the page has been torn down, and then #chScrollBtn is no longer in the DOM. The line has not changed since the initial import.

Fix direction:

• Null-check the button, or capture it once next to msgEl and guard against it being detached.
• Check whether the listener should also be removed on page destroy, so it does not accumulate. That is the same leak class as Follow-ups to #255: expanded group dead end on mobile, nodesEsc listener leak, two test gaps #259/Follow-ups to #268, #260 and #212: pktEsc listener leak, test gaps and comment fixes #282.

Acceptance:

• No console error when navigating #/channels → #/live → #/channels several times.
• A unit test that dispatches scroll on a detached #chMessages without throwing.

2. Two mobile tap targets below 48 px

Measured at 375×812 with a touch user agent:

Neither was in the scope of #239/#264. Raise both to the 48 px minimum under pointer: coarse, using the same pattern as #239. Extend test-touch-targets.js to cover both elements.

Low priority.