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/keyboard-accessory.js

@@ -300,7 +300,10 @@ const KeyboardAccessoryBar = {
     const sendText = () => {
       const text = textarea.value;
       close();
-      if (text) app.sendInput(text);
+      if (text) {
+        app.sendInput(text);
+        setTimeout(() => app.sendInput('\r'), 80);
+      }
     };
 
     // Filter to images, close the dialog, and hand off to the shared