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/styles.css

@@ -9307,9 +9307,9 @@ kbd {
   bottom: calc(var(--safe-area-bottom) + 40px);
   z-index: 52;
   display: block;
-  min-height: 44px;
-  max-height: 96px;
-  padding: 12px 10px;
+  min-height: 34px;
+  max-height: 68px;
+  padding: 6px 10px;
   border: 1px solid rgba(80, 120, 190, 0.55);
   border-left: none;
   border-right: none;