feat(terminal): Chinese / IME input (P3)

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>

Cargo.toml

@@ -42,6 +42,7 @@ web-sys = { version = "0.3", default-features = false, features = [
     "Clipboard",
     "ClipboardEvent",
     "CloseEvent",
+    "CompositionEvent",
     "console",
     "DataTransfer",
     "Document",
@@ -52,6 +53,7 @@ web-sys = { version = "0.3", default-features = false, features = [
     "Event",
     "EventTarget",
     "HtmlElement",
+    "InputEvent",
     "KeyboardEvent",
     "Location",
     "MessageEvent",