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/input-cjk.js

@@ -41,8 +41,6 @@
 // eslint-disable-next-line no-unused-vars
 const CjkInput = (() => {
   let _textarea = null;
-  let _terminalContainer = null;
-  let _xtermTextarea = null;
   let _send = null;
   let _initialized = false;
   let _composing = false;
@@ -77,23 +75,6 @@ const CjkInput = (() => {
     _textarea.setSelectionRange(1, 1);
   }
 
-  function _isMobileComposer() {
-    return !!(
-      _textarea &&
-      typeof MobileDetection !== 'undefined' &&
-      MobileDetection.isTouchDevice() &&
-      _textarea.classList.contains('cjk-input-visible')
-    );
-  }
-
-  function _resetInput() {
-    if (_isMobileComposer()) {
-      _textarea.value = '';
-    } else {
-      _resetToPhantom();
-    }
-  }
-
   /** Check if textarea contains only phantom(s) or is empty — no real user text */
   function _isEffectivelyEmpty() {
     return !_strip(_textarea.value);
@@ -116,44 +97,22 @@ const CjkInput = (() => {
       _composing = false;
       _textarea = document.getElementById('cjkInput');
       if (!_textarea) return this;
-      _terminalContainer = document.getElementById('terminalContainer');
 
-      // Seed the phantom character for the hidden/immediate CJK path.
-      _resetInput();
+      _resetToPhantom();
 
       _listeners.mousedown = (e) => { e.stopPropagation(); };
       _listeners.focus = () => {
         window.cjkActive = true;
-        if (_isMobileComposer() && _textarea.value === PHANTOM) {
-          _textarea.value = '';
-          return;
-        }
-        // Restore phantom if textarea was emptied while blurred
-        if (!_textarea.value && !_isMobileComposer()) _resetToPhantom();
+        if (!_textarea.value) _resetToPhantom();
       };
       _listeners.blur = () => { window.cjkActive = false; };
       _textarea.addEventListener('mousedown', _listeners.mousedown);
       _textarea.addEventListener('focus', _listeners.focus);
       _textarea.addEventListener('blur', _listeners.blur);
 
-      _listeners.xtermFocusRedirect = () => {
-        if (!_isMobileComposer()) return;
-        _textarea.focus();
-      };
-      if (_terminalContainer) {
-        _xtermTextarea = _terminalContainer.querySelector('.xterm-helper-textarea');
-        if (_xtermTextarea) {
-          _xtermTextarea.addEventListener('focus', _listeners.xtermFocusRedirect, { capture: true });
-        }
-      }
-
       // ── Composition tracking ──
       _listeners.compositionstart = () => {
         _composing = true;
-        if (_isMobileComposer()) {
-          if (_textarea.value === PHANTOM) _textarea.value = '';
-          return;
-        }
         // Clear phantom so IME sees a clean textarea — some IMEs include
         // existing text in the composition region which would corrupt input.
         if (_textarea.value === PHANTOM) {
@@ -162,7 +121,6 @@ const CjkInput = (() => {
       };
       _listeners.compositionend = () => {
         _composing = false;
-        if (_isMobileComposer()) return;
         // Defer flush: some Android IMEs haven't committed text to textarea
         // when compositionend fires. setTimeout(0) ensures we read the final value.
         setTimeout(_flush, 0);
@@ -185,7 +143,7 @@ const CjkInput = (() => {
           } else {
             _send('\r');
           }
-          _resetInput();
+          _resetToPhantom();
           return;
         }
 
@@ -193,7 +151,7 @@ const CjkInput = (() => {
         if (e.key === 'Escape') {
           e.preventDefault();
           _composing = false;
-          _resetInput();
+          _resetToPhantom();
           return;
         }
 
@@ -207,19 +165,6 @@ const CjkInput = (() => {
         // Below: only when NOT composing (composing keystrokes belong to IME)
         if (_composing) return;
 
-        if (_isMobileComposer()) {
-          if (e.key === 'Backspace' && _isEffectivelyEmpty()) {
-            e.preventDefault();
-            _send('\x7f');
-            return;
-          }
-          if (PASSTHROUGH_KEYS[e.key] && _isEffectivelyEmpty()) {
-            e.preventDefault();
-            _send(PASSTHROUGH_KEYS[e.key]);
-          }
-          return;
-        }
-
         // Backspace: forward to PTY when no real text in textarea
         // (Desktop path — Android uses the input event + phantom approach)
         if (e.key === 'Backspace' && _isEffectivelyEmpty()) {
@@ -251,13 +196,6 @@ const CjkInput = (() => {
       // making keydown unreliable. input fires AFTER character insertion and
       // carries inputType which tells us whether the text is final or tentative.
       _listeners.input = (e) => {
-        if (_isMobileComposer()) {
-          if (_textarea.value.includes(PHANTOM)) {
-            _textarea.value = _strip(_textarea.value);
-          }
-          return;
-        }
-
         // ── Backspace / delete detection ──
         // Android long-press backspace generates rapid deleteContentBackward events.
         // The phantom character ensures the textarea is never truly empty, so each
@@ -305,13 +243,8 @@ const CjkInput = (() => {
           if (handler) _textarea.removeEventListener(event, handler);
         }
       }
-      if (_xtermTextarea && _listeners.xtermFocusRedirect) {
-        _xtermTextarea.removeEventListener('focus', _listeners.xtermFocusRedirect, { capture: true });
-      }
       window.cjkActive = false;
       _composing = false;
-      _terminalContainer = null;
-      _xtermTextarea = null;
       for (const key of Object.keys(_listeners)) delete _listeners[key];
       _initialized = false;
     },