fix(mobile): restore real-time CJK input and terminal tap interaction

Root cause: the mobile-composer mode (02fa3f3) routed CJK text through
local-echo buffering, which accumulated characters until Enter instead
of sending each composed word to the PTY immediately. Additionally,
xtermFocusRedirect hijacked all terminal taps, preventing cursor
positioning and scroll interaction.

Changes:
- Remove mobile-composer accumulation mode from input-cjk.js — all
  platforms now use the same immediate-flush path (compositionend →
  flush → PTY)
- Bypass local-echo buffering in _handleCjkInput (terminal-ui.js) —
  the CJK textarea already provides visual feedback
- Remove xtermFocusRedirect so terminal taps work normally again
- Reduce CJK textarea height (34px min, 6px padding) for less
  screen intrusion
- Paste dialog now sends Enter after text so pasted content submits
- Hide CJK textarea on welcome screen (no active session)
- Add Opus 4.6 model options to selector

src/web/public/app.js

@@ -2206,7 +2206,6 @@ class CodemanApp {
     document.body.classList.toggle('cjk-input-visible', !!showCjk);
     cjkEl.style.display = showCjk ? 'block' : 'none';
     cjkEl.setAttribute('aria-hidden', showCjk ? 'false' : 'true');
-    if (showCjk && cjkEl.value === '\u200B') cjkEl.value = '';
     if (!showCjk) window.cjkActive = false;
     if (typeof KeyboardHandler !== 'undefined') KeyboardHandler.updateLayoutForKeyboard();
   }