Korean (Hangul) IME input not working

## Description

Korean (Hangul) IME input is not working in ghostty-web 0.4.0.

While PR #90 added support for Chinese and Japanese IME input, Korean IME behaves differently and doesn't work properly.

## Steps to Reproduce

1. Visit the official demo at https://ghostty.ondis.co
2. Switch to Korean input method (e.g., fcitx5-hangul on Linux, or macOS Korean keyboard)
3. Try typing Korean characters like "안녕하세요"

## Expected Behavior

Korean characters should be composed and displayed in the terminal, similar to how Chinese/Japanese IME works.

## Actual Behavior

Korean IME composition doesn't work. Characters are not displayed or composition events are not handled properly.

## Environment

- OS: Linux (tested), likely affects all platforms
- Browser: Chrome/Firefox
- ghostty-web version: 0.4.0

## Technical Notes

Korean IME differs from Chinese/Japanese:
- Korean uses a unique syllable block composition system (Jamo: 자음+모음+자음)
- The composition process involves combining initial consonant + vowel + optional final consonant
- During composition, partially formed characters (e.g., ㄱ → 가 → 간) need to be displayed

The current `handleCompositionEnd` in InputHandler may not properly handle Korean composition events.

## Related

- PR #90 added CJK support but may have missed Korean-specific testing
- Issue #97 reports CJK input position issues (different from this input issue)