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>

style/tailwind.css

@@ -610,13 +610,32 @@
     scrollbar-color: rgb(148 163 184 / 60%) transparent;
   }
 
-  .terminal-screen:focus {
+  .terminal-screen:focus-within {
     border-color: #38bdf8;
     box-shadow:
       inset 0 1px 0 rgb(255 255 255 / 4%),
       0 0 0 3px rgb(56 189 248 / 15%);
   }
 
+  .terminal-ime {
+    position: absolute;
+    z-index: 100;
+    opacity: 0;
+    background: transparent;
+    color: transparent;
+    caret-color: transparent;
+    border: none;
+    outline: none;
+    resize: none;
+    padding: 0;
+    margin: 0;
+    overflow: hidden;
+    pointer-events: auto;
+    font-family: inherit;
+    font-size: 1px;
+    line-height: 1px;
+  }
+
   .terminal-text {
     margin: 0;
     min-height: 100%;