2.1.23

Remove MosaicMobileSession floating top-right pill

The body-mounted Mithril root rendered an avatar (or "Log In" pill for
guests) at the top-right of the phone viewport, escaping the
#drawer.App-drawer transform trap. The guest pill was reading as a
grayed-out / non-interactive label rather than an actionable button,
and duplicated the session affordance that already lives in the
hamburger drawer.

Mobile session controls now use Flarum 2's intended pattern: tap the
hamburger to access Log In / Sign Up / Notifications / user dropdown
inside the drawer.

Deletes:
- js/src/forum/components/MobileSessionAvatar.js
- Mount logic in forum/index.js
- .MosaicMobileSession / .MosaicMobileSession-trigger /
  .MosaicMobileSession-loginLabel rules in less/header.less

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>