fix: mobile CJK input, terminal flicker, and layout overflow

Terminal flicker:
- Skip buffer-recovered/clear-terminal events during active buffer load
  to prevent competing clear+rewrite cycles (app.js)
- Move viewport+scrollback clear inside dimension-change guard so resize
  without actual SIGWINCH doesn't blank the terminal (terminal-ui.js)
- Sync _lastResizeDims on explicit resize to prevent redundant clears

CJK input rewrite (input-cjk.js):
- Use InputEvent.inputType to distinguish insertText (final) from
  insertCompositionText (tentative) — fixes Chinese punctuation and
  English text being swallowed during Android IME composition
- Remove isComposing guard on Enter so it always sends
- Phantom character (U+200B) keeps textarea non-empty so Android
  long-press backspace generates continuous deleteContentBackward
  events at the keyboard's native repeat rate

CJK input settings:
- Add "CJK Input" toggle in Settings > Input (index.html, settings-ui.js)
- Store as device-specific setting (cjkInputEnabled), not synced to server
- Replace INPUT_CJK_FORM env var dependency with user-controlled setting
  (env var still works as server override)

Mobile layout:
- Fix welcome screen overflow on phones by constraining .welcome-content
  to calc(100vw - 1.5rem) (mobile.css)
- Move xterm helper textarea on-screen for touch devices to fix iOS
  keyboard input (styles.css)
- Focus terminal synchronously in user-gesture context for iOS Safari
  keyboard activation (session-ui.js, app.js)
- Refocus terminal on tap (not scroll) in touch handler (terminal-ui.js)

src/web/public/styles.css

@@ -94,7 +94,11 @@ textarea:focus-visible {
   outline: none;
 }
 
-/* xterm.js hidden textarea — must never show focus ring or caret */
+/* xterm.js hidden textarea — must never show focus ring or caret.
+ * On mobile (touch devices), override xterm.css defaults that position the
+ * textarea at left:-9999em with width/height:0. iOS Safari ignores keyboard
+ * input to off-screen zero-size textareas, so we move it on-screen with a
+ * minimal size while keeping it visually invisible. */
 .xterm-helper-textarea {
   border: none !important;
   box-shadow: none !important;
@@ -102,6 +106,14 @@ textarea:focus-visible {
   opacity: 0 !important;
   caret-color: transparent !important;
 }
+.touch-device .xterm .xterm-helper-textarea {
+  left: 0 !important;
+  top: 0 !important;
+  width: 1px !important;
+  height: 1px !important;
+  z-index: -1 !important;
+  font-size: 16px !important; /* prevent iOS auto-zoom on focus */
+}
 
 /* Session tab focus */
 .session-tab:focus-visible {