feat: add CJK IME input textarea with env toggle

Add a dedicated textarea below the terminal for CJK (Korean/Japanese/Chinese)
IME input. xterm.js intercepts IME composition events, preventing composed
characters from displaying correctly. This textarea bypasses xterm entirely
by using native browser IME handling — text accumulates until Enter, then
sends to PTY in one shot.

- Always-visible textarea below terminal (inside .terminal-wrap flex column)
- focus/blur sets window.cjkActive flag to block xterm onData
- Enter sends textarea.value + \r to PTY, Escape clears
- Arrow keys, Ctrl+C/D/L/Z, Tab, Backspace pass through to PTY when empty
- attachCustomKeyEventHandler suppresses xterm key handling during composition
- INPUT_CJK_FORM=ON|OFF env var toggle (default: off, passed via SSE init)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/web/public/input-cjk.js

@@ -0,0 +1,92 @@
+/**
+ * @fileoverview CJK IME input for xterm.js terminal.
+ *
+ * Always-visible textarea below the terminal (in index.html).
+ * The browser handles IME composition natively — we just read
+ * textarea.value on Enter and send it to PTY.
+ * While this textarea has focus, window.cjkActive = true blocks xterm's onData.
+ * Arrow keys and function keys are forwarded to PTY directly.
+ *
+ * @globals {object} CjkInput
+ * @loadorder 5.5 of 9 — loaded after keyboard-accessory.js, before app.js
+ */
+
+// eslint-disable-next-line no-unused-vars
+const CjkInput = (() => {
+  let _textarea = null;
+  let _send = null;
+
+  const PASSTHROUGH_KEYS = {
+    ArrowUp:    '\x1b[A',
+    ArrowDown:  '\x1b[B',
+    ArrowLeft:  '\x1b[D',
+    ArrowRight: '\x1b[C',
+    Home:       '\x1b[H',
+    End:        '\x1b[F',
+    Tab:        '\t',
+  };
+
+  const CTRL_KEYS = {
+    c: '\x03', d: '\x04', l: '\x0c', z: '\x1a', a: '\x01', e: '\x05',
+  };
+
+  return {
+    init({ send }) {
+      _send = send;
+      _textarea = document.getElementById('cjkInput');
+      if (!_textarea) return this;
+
+      _textarea.addEventListener('mousedown', (e) => { e.stopPropagation(); });
+      _textarea.addEventListener('focus', () => { window.cjkActive = true; });
+      _textarea.addEventListener('blur', () => { window.cjkActive = false; });
+
+      _textarea.addEventListener('keydown', (e) => {
+        if (e.isComposing || e.keyCode === 229) return;
+
+        // Enter: send accumulated text (or bare Enter if empty)
+        if (e.key === 'Enter') {
+          e.preventDefault();
+          if (_textarea.value) {
+            _send(_textarea.value + '\r');
+            _textarea.value = '';
+          } else {
+            _send('\r');
+          }
+          return;
+        }
+
+        // Escape: clear textarea
+        if (e.key === 'Escape') {
+          e.preventDefault();
+          _textarea.value = '';
+          return;
+        }
+
+        // Ctrl combos: forward to PTY
+        if (e.ctrlKey && CTRL_KEYS[e.key]) {
+          e.preventDefault();
+          _send(CTRL_KEYS[e.key]);
+          return;
+        }
+
+        // Backspace: delete from textarea if has text, else forward to PTY
+        if (e.key === 'Backspace' && !_textarea.value) {
+          e.preventDefault();
+          _send('\x7f');
+          return;
+        }
+
+        // Arrow/function keys: forward to PTY when textarea is empty
+        if (PASSTHROUGH_KEYS[e.key] && !_textarea.value) {
+          e.preventDefault();
+          _send(PASSTHROUGH_KEYS[e.key]);
+          return;
+        }
+      });
+
+      return this;
+    },
+
+    get element() { return _textarea; },
+  };
+})();