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/app.js

@@ -886,6 +886,8 @@ class CodemanApp {
     // Server sends this after SSE backpressure clears — terminal data was dropped,
     // so reload the buffer to recover from any display corruption.
     if (!this.activeSessionId || !this.terminal) return;
+    // Skip if buffer load already in progress — avoids competing clear+rewrite cycles
+    if (this._isLoadingBuffer) return;
     try {
       const res = await fetch(`/api/sessions/${this.activeSessionId}/terminal?tail=${TERMINAL_TAIL_SIZE}`);
       const data = await res.json();
@@ -909,6 +911,12 @@ class CodemanApp {
 
   async _onSessionClearTerminal(data) {
     if (data.id === this.activeSessionId) {
+      // Skip if selectSession is already loading the buffer — clearTerminal arriving
+      // during buffer load would clear the terminal mid-write, causing visible flicker
+      // and a race between two concurrent chunkedTerminalWrite calls (especially on mobile
+      // where rAF is slower). selectSession will handle the final buffer state.
+      if (this._isLoadingBuffer) return;
+
       // Fetch buffer, clear terminal, write buffer, resize (no Ctrl+L needed)
       try {
         const res = await fetch(`/api/sessions/${data.id}/terminal`);
@@ -1299,6 +1307,16 @@ class CodemanApp {
     });
   }
 
+  /** Show/hide the CJK input textarea based on user setting or server override */
+  _updateCjkInputState() {
+    const cjkEl = document.getElementById('cjkInput');
+    if (!cjkEl) return;
+    const settings = this.loadAppSettingsFromStorage();
+    const showCjk = this._serverCjkOverride || settings.cjkInputEnabled || false;
+    cjkEl.style.display = showCjk ? 'block' : 'none';
+    if (!showCjk) window.cjkActive = false;
+  }
+
   handleInit(data) {
     // Clear the init fallback timer since we got data
     if (this._initFallbackTimer) {
@@ -1307,12 +1325,9 @@ class CodemanApp {
     }
     const gen = ++this._initGeneration;
 
-    // CJK input form: show/hide based on server env INPUT_CJK_FORM=ON
-    const cjkEl = document.getElementById('cjkInput');
-    if (cjkEl) {
-      cjkEl.style.display = data.inputCjkForm ? 'block' : 'none';
-      if (!data.inputCjkForm) window.cjkActive = false;
-    }
+    // CJK input form: controlled by user setting (with server env as override)
+    this._serverCjkOverride = data.inputCjkForm || false;
+    this._updateCjkInputState();
 
     // Update version displays (header and toolbar)
     if (data.version) {
@@ -1988,6 +2003,12 @@ class CodemanApp {
 
   async selectSession(sessionId) {
     if (this.activeSessionId === sessionId) return;
+    // Focus terminal SYNCHRONOUSLY before any await — iOS Safari only honors
+    // programmatic focus() within the user-gesture call stack (e.g. tab click).
+    // After the first await the gesture context is lost and focus() is silently
+    // ignored, leaving the keyboard unable to send input to the terminal.
+    if (this.terminal) this.terminal.focus();
+
     const _selStart = performance.now();
     const _selName = this.sessions.get(sessionId)?.name || sessionId.slice(0,8);
     _crashDiag.log(`SELECT: ${_selName}`);