Implement IME composition support and migrate regular character input to
a hidden textarea, matching the xterm.js approach.
Key changes:
- Add a hidden, absolutely-positioned `<textarea>` inside `.terminal-screen`
that follows the terminal cursor cell. It receives focus on click, connect,
and resize so the OS/browser IME candidate window appears at the cursor.
- `on:input` now sends regular printable characters; `on:keydown` only handles
special keys (arrows, Enter, Esc, Backspace, F-keys, Ctrl combos).
- `on:compositionstart/end` track IME sessions and send the final composed
string at the end, avoiding partial pinyin/jamo/kana leaks.
- Move `on:paste` and `on:keydown` from `.terminal-screen` to the textarea so
focus stays on the IME target.
- Replace `.terminal-screen:focus` with `:focus-within` since the screen div
no longer holds focus.
- Add `is_regular_text_input` helper and unit test.
Web-sys features extended with `InputEvent` and `CompositionEvent`.
Tests: 24/24 pass, clippy clean.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
f76d341501