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

[TeigenZhang]

Summary

• Terminal flicker fix: prevent buffer-load race conditions and redundant viewport clears during resize — terminal no longer flashes blank on mobile
• CJK input rewrite: use InputEvent.inputType to handle Android IME composition correctly — Chinese punctuation, English text, and long-press backspace all work now
• Settings UI: add "CJK Input" toggle (Settings > Input) so users can enable/disable the dedicated IME input field without server env vars
• Mobile layout: fix welcome screen horizontal overflow on phones (<430px)

Details

Terminal flicker (app.js, terminal-ui.js)

• Skip buffer-recovered / clear-terminal SSE events while _isLoadingBuffer is true
• Move \x1b[3J\x1b[H\x1b[2J (scrollback+viewport clear) inside the dimension-change guard — only clear when SIGWINCH is actually sent
• Sync _lastResizeDims on explicit resize to prevent double-clear

CJK input (input-cjk.js)

• inputType dispatch: insertText during composition → flush immediately (punctuation, confirmed words); insertCompositionText → wait for compositionend (pinyin candidates)
• Enter key: removed !e.isComposing guard + force-reset _composing — Enter always sends on Android
• Phantom character (U+200B): textarea always contains a zero-width space so Android long-press backspace generates continuous deleteContentBackward events at native repeat rate
• Composition start clears phantom; flush restores it

CJK settings (index.html, settings-ui.js, app.js)

• New cjkInputEnabled toggle in Settings > Input section
• Device-specific (not synced to server) — mobile and desktop can have different defaults
• INPUT_CJK_FORM=ON env var still works as server-side override

Mobile layout (mobile.css, styles.css, session-ui.js)

• .welcome-content: max-width: calc(100vw - 1.5rem) prevents horizontal overflow
• .touch-device .xterm-helper-textarea: move on-screen (1px×1px) so iOS Safari accepts keyboard input
• Synchronous terminal.focus() in user-gesture context for iOS keyboard activation
• Tap-to-refocus on terminal touch area

Test plan

• Mobile (Android + Chinese IME): verify Chinese characters, punctuation (，。！), and English text all input correctly with CJK Input enabled
• Mobile: long-press backspace deletes continuously
• Mobile: welcome screen fits within viewport (no horizontal scroll)
• Mobile: terminal doesn't flicker on tab switch or resize
• Desktop: CJK Input toggle off by default, no regressions
• Settings > Input: CJK Input toggle shows/hides the textarea immediately on save

🤖 Generated with Claude Code

[Ark0N]

Thorough review, no issues found. Clean, well-documented work across all 8 files.

Highlights:

• inputType dispatch in input-cjk.js correctly handles Android IME semantics — insertText vs insertCompositionText is the right discriminator
• Phantom character (U+200B) for Android long-press backspace is clever and _flush() idempotency prevents double-sends from deferred compositionend
• Terminal flicker fix properly gates the clear sequence behind the dimension-change guard — no more blank flashes from clearing without a subsequent Ink redraw
• iOS Safari focus pattern (synchronous before await) applied consistently across selectSession, runClaude, and runOpenCode

Thank you very much for the many contributions already — this is excellent mobile work. 🎯