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/settings-ui.js

@@ -364,6 +364,7 @@ Object.assign(CodemanApp.prototype, {
     document.getElementById('appSettingsTunnelEnabled').checked = settings.tunnelEnabled ?? false;
     this.loadTunnelStatus();
     document.getElementById('appSettingsLocalEcho').checked = settings.localEchoEnabled ?? MobileDetection.isTouchDevice();
+    document.getElementById('appSettingsCjkInput').checked = settings.cjkInputEnabled ?? false;
     document.getElementById('appSettingsTabTwoRows').checked = settings.tabTwoRows ?? defaults.tabTwoRows ?? false;
     // Claude CLI settings
     const claudeModeSelect = document.getElementById('appSettingsClaudeMode');
@@ -1180,6 +1181,7 @@ Object.assign(CodemanApp.prototype, {
       imageWatcherEnabled: document.getElementById('appSettingsImageWatcherEnabled').checked,
       tunnelEnabled: document.getElementById('appSettingsTunnelEnabled').checked,
       localEchoEnabled: document.getElementById('appSettingsLocalEcho').checked,
+      cjkInputEnabled: document.getElementById('appSettingsCjkInput').checked,
       tabTwoRows: document.getElementById('appSettingsTabTwoRows').checked,
       // Claude CLI settings
       claudeMode: document.getElementById('appSettingsClaudeMode').value,
@@ -1296,9 +1298,12 @@ Object.assign(CodemanApp.prototype, {
     this.renderProjectInsightsPanel();  // Re-render to apply visibility setting
     this.updateSubagentWindowVisibility();  // Apply subagent window visibility setting
 
+    // Apply CJK input visibility immediately
+    this._updateCjkInputState();
+
     // Save to server (includes notification prefs for cross-browser persistence)
-    // Strip device-specific keys — localEchoEnabled is per-platform (touch default differs)
-    const { localEchoEnabled: _leo, ...serverSettings } = settings;
+    // Strip device-specific keys — localEchoEnabled/cjkInputEnabled are per-platform
+    const { localEchoEnabled: _leo, cjkInputEnabled: _cjk, ...serverSettings } = settings;
     try {
       await this._apiPut('/api/settings', { ...serverSettings, notificationPreferences: notifPrefsToSave, voiceSettings });
 
@@ -1682,7 +1687,7 @@ Object.assign(CodemanApp.prototype, {
         const displayKeys = new Set([
           'showFontControls', 'showSystemStats', 'showTokenCount', 'showCost',
           'showMonitor', 'showProjectInsights', 'showFileBrowser', 'showSubagents',
-          'subagentActiveTabOnly', 'tabTwoRows', 'localEchoEnabled',
+          'subagentActiveTabOnly', 'tabTwoRows', 'localEchoEnabled', 'cjkInputEnabled',
         ]);
         // Merge settings: non-display keys always sync from server,
         // display keys only seed from server when localStorage has no value