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/mobile.css

@@ -1147,6 +1147,7 @@ html.mobile-init .file-browser-panel {
 
   /* Compact welcome overlay for mobile */
   .welcome-content {
+    max-width: calc(100vw - 1.5rem);
     padding: 1rem 0.75rem;
   }