fix(mobile): merge-time fixes for the prompt composer (#444)

- styles.css: restate the composer overlay's own bottom gutter after the fold rules
  (the generic .paste-overlay longhand erased it: 0px flat, hinge strip replacing it
  folded) and subtract the fold strip from the dialog's max-height
- test/foldable-layout.test.ts: simulate the cascade for
  .paste-overlay.prompt-composer-overlay (fails without the CSS fix); pin the palette
  anchor by name instead of ELEMENTS.at(-1)
- keyboard-accessory.js: guard the app global in refreshForActiveSession() like the
  rest of the file
- keyboard-accessory.js: a whitespace-only draft is empty (Send no longer submits
  blank lines); the text still goes out untrimmed
- keyboard-accessory.js: derive _composerMaxLength and the frame refusal from one
  64 KiB frame limit minus both bracketed-paste markers so they cannot drift
- keyboard-accessory.js: translate the textarea placeholder and label at build time,
  since the DOM translator skips <textarea> subtrees
- i18n.js: zh-CN entries for the composer dialog copy
- docs/wiki/Mobile-Guide.md: describe the Compose key instead of a clipboard key
- CLAUDE.md: a "Mobile prompt composer" paragraph after the accessory bar one
- test/mobile-prompt-composer.test.ts: pin the whitespace rule and the derived budget

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
(cherry picked from commit f6725ba52da17b0bdbee8be3b5011e7cae514f69)

docs/wiki/Mobile-Guide.md

@@ -60,14 +60,20 @@ On by default; it can be turned off in settings.
 
 A row of keys above the virtual keyboard, and what it contains depends on the session.
 
-**Agent sessions** get quick actions: `/init`, `/clear`, `/compact`, a clipboard key, `Esc`,
-a path picker, an image key, and 🧠 when Read My Mind is on. Destructive commands need a
-double press, so you cannot fire `/clear` with a stray thumb. On Codex sessions the bar also
-shows `⇧←` and `⇧→`, the Shift-modified arrows Codex binds to editing the last queued
-message and walking the prompt stack.
+**Agent sessions** get quick actions: `/init`, `/clear`, `/compact`, a Compose key, `Esc`,
+a path picker, and 🧠 when Read My Mind is on. Compose opens a multiline editor with
+autocorrect: Enter adds a new line, and only Send delivers the text, as one paste followed
+by Enter, so your line breaks reach the agent intact. Anything already typed on the terminal
+prompt moves into the editor when it opens. Drafts are kept per session and in memory only,
+so switching tabs keeps them and a page reload forgets them; a dot on the key shows a draft
+is parked. The editor's Image button attaches photos and puts their paths into the draft.
+Destructive commands need a double press, so you cannot fire `/clear` with a stray thumb. On
+Codex sessions the bar also shows `⇧←` and `⇧→`, the Shift-modified arrows Codex binds to
+editing the last queued message and walking the prompt stack.
 
 **Shell sessions** automatically swap it for terminal controls: `Ctrl`, `Esc`, `Tab`, four
-arrows, paste, and dismiss. Your normal preference is remembered and restored when you
+arrows, a direct Paste key (shell input is not an agent prompt, so there is no Compose
+there), and dismiss. Your normal preference is remembered and restored when you
 switch back to an agent session, so a settings change during a shell session cannot strip
 the bar away permanently.