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/server.ts

@@ -1947,6 +1947,7 @@ export class WebServer extends EventEmitter {
       globalStats: this.store.getAggregateStats(activeSessionTokens),
       subagents: subagentWatcher.getRecentSubagents(15), // 15 min to avoid stale agents
       timestamp: now,
+      inputCjkForm: process.env.INPUT_CJK_FORM?.toUpperCase() === 'ON',
     };
 
     this.cachedLightState = { data: result, timestamp: now };