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)

src/web/public/styles.css

@@ -13809,7 +13809,10 @@ body.touch-device.cjk-input-visible .main {
 
 .prompt-composer-dialog {
   max-width: 560px;
-  max-height: calc(100dvh - min(15dvh, 72px) - 12px - env(safe-area-inset-bottom));
+  /* The overlay's top inset and bottom gutter, plus the strip a folding device
+     reserves below the dialog (0px everywhere else; see the fold rules at the
+     end of this file). */
+  max-height: calc(100dvh - min(15dvh, 72px) - 12px - env(safe-area-inset-bottom) - var(--fold-block-end));
   overflow-y: auto;
 }
 
@@ -18683,6 +18686,19 @@ html[data-session-list="sidebar"][data-sidebar="collapsed"] .btn-sidebar-toggle
   padding-bottom: var(--fold-block-end);
 }
 
+/* The mobile prompt composer is a .paste-overlay with a gutter of its own: a
+   three-value `padding` shorthand whose bottom is 12px plus the safe area. The
+   generic rule above is a later longhand at the same specificity, so it ERASED
+   that gutter (measured at 393x852: padding-bottom 0 instead of 12px, and with
+   the fold variables set the hinge strip stood in for the gutter instead of
+   adding to it). Restate the composer's bottom gutter on top of the strip; the
+   side has no gutter of its own. The dialog's height cap subtracts the same
+   strip where it is declared (.prompt-composer-dialog). */
+.prompt-composer-overlay {
+  padding-right: var(--fold-inline-end);
+  padding-bottom: calc(12px + env(safe-area-inset-bottom) + var(--fold-block-end));
+}
+
 /* The response viewer is a bottom sheet, so a vertical hinge running through it
    is fine, since it is a wide surface like the terminal and inset dialogs are what
    the fold guidance is about. A horizontal hinge is not: in tabletop pose the