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/index.html

@@ -231,7 +231,11 @@
 
     <!-- Main Terminal Area -->
     <main class="main">
-      <div class="terminal-container" id="terminalContainer"></div>
+      <div class="terminal-wrap">
+        <div class="terminal-container" id="terminalContainer"></div>
+        <textarea id="cjkInput" rows="1" placeholder="CJK 입력 → Enter 전송 / ESC 취소"
+          autocomplete="off" autocorrect="off" autocapitalize="off" spellcheck="false"></textarea>
+      </div>
 
       <!-- Welcome Overlay (shown when no session active) -->
       <div class="welcome-overlay" id="welcomeOverlay">
@@ -1689,6 +1693,7 @@
   <script defer src="voice-input.js"></script>
   <script defer src="notification-manager.js"></script>
   <script defer src="keyboard-accessory.js"></script>
+  <script defer src="input-cjk.js"></script>
   <script defer src="app.js"></script>
   <script defer src="ralph-wizard.js"></script>
   <script defer src="api-client.js"></script>