Opus advisor caught a recoverable footgun in PR #1441's manual flag: if
focus is lost mid-composition (window blur or older Safari WebKit IME
quirk), compositionend may never fire and _imeComposing stays true
until the next full composition cycle. Result: Enter-to-send is
silently broken until page reload — an unrecoverable stuck state for
something that's supposed to be transient.
Add a blur listener that also resets the flag. Cheap belt-and-suspenders
against the stuck state. Adds 1 regression test pinning the listener.
(other Opus findings logged in /tmp/stage-264-brief.md as follow-up
issues: _LOGIN_LOCALE parity for ja/pt/ko, promote _isImeEnter to the
6 other Safari-affected Enter guards in sessions.js + ui.js)
e6e9868625
