fix: Korean/CJK IME input and rendering in Sidebar Terminal

Fixes #1272

This commit addresses three separate Korean/CJK bugs in the Sidebar Terminal:

**Bug 1 - IME Input**: Korean text typed via IME composition was not
reaching the PTY correctly. Added compositionstart/compositionend event
listeners to suppress partial jamo fragments and only send the final
composed string.

**Bug 2a - Font Rendering**: Added CJK monospace font fallbacks
("Noto Sans Mono CJK KR", "Malgun Gothic") to both the xterm.js
fontFamily config and the CSS --font-mono variable. This ensures
consistent cell-width calculations for Korean characters.

**Bug 2b - UTF-8 Boundary Detection**: Added buffering logic to prevent
multi-byte UTF-8 characters (Korean is 3 bytes) from being split across
WebSocket chunks. This follows the same pattern as PR #1007 which fixed
the sidebar-agent path, but extends it to the terminal-agent path.

Special thanks to @ldybob for the excellent root cause analysis and
proposed solutions in issue #1272.

Tested on WSL2 + Windows 11 with Korean IME.

extension/sidepanel.css

@@ -38,7 +38,7 @@
 
   /* Typography */
   --font-system: -apple-system, BlinkMacSystemFont, 'Segoe UI', system-ui, sans-serif;
-  --font-mono: 'JetBrains Mono', 'SF Mono', 'Fira Code', 'Cascadia Code', monospace;
+  --font-mono: 'JetBrains Mono', 'SF Mono', 'Fira Code', 'Cascadia Code', 'Noto Sans Mono CJK KR', 'Malgun Gothic', monospace;
 
   /* Radius */
   --radius-sm: 4px;