fix(extension): defer composing reset to stop CJK IME double-send

[0xteng]

Problem

Typing CJK (Chinese / Japanese) through an IME in the sidebar terminal chat
sends every committed segment twice. Typing "在这里我可以做什么" renders as
"在在这里这里我我可以可以做做什么什么".

Root cause

The IME composition handling added for the Korean work (#1272, PR #1297) lives
in extension/sidepanel-terminal.js, but it clears composing = false
synchronously inside the compositionend handler, before sending.

Browsers fire the trailing input event (→ xterm term.onData) immediately
after compositionend, carrying the same composed string. By then composing
is already false, so the if (composing) return; guard in onData no longer
suppresses it, and the text is ws.send-ed a second time.

Korean composition (tested in #1297) doesn't surface this, but languages that
commit per-segment and fire a trailing input event — Chinese, Japanese — get
every segment doubled.

Fix

Defer the composing = false reset to the next tick with setTimeout(…, 0),
so the onData event that fires right after compositionend is still
suppressed. One-line behavioral change; the Korean path from #1297 is
unaffected.

Testing

Manually verified in the GStack Browser sidebar (v1.44). Before:
"在这里我可以做什么" rendered as "在在这里这里我我可以可以做做什么什么".
After the fix: renders once, correctly. English input/output unaffected.