[Feature]: Auto-expanding multiline chat composer textarea (OpenClaw parity)

[G7CNF]

Summary

Make the web chat composer auto-expand vertically for multiline input/paste, up to a sensible max height/rows, matching OpenClaw behavior.

Motivation

OpenClaw expands input as users paste/type longer prompts. In ZeroClaw the fixed-height input makes long prompts harder to edit.

Expected behavior

• Composer grows automatically as content wraps/newlines are added.
• Maintains min height and caps at max height/rows with internal scroll past max.
• Works on paste, typing, deletion, and window resize.
• Keeps send behavior intact (Enter vs Shift+Enter semantics as currently defined).

Accessibility/UX

• No layout jump that obscures latest messages.
• Works on desktop and mobile widths.
• Preserves cursor position during resize.

Acceptance criteria

• Input expands across several lines for multiline prompts.
• No regression in submit key handling.
• Stable performance while typing rapidly/pasting large text.

Parity target

OpenClaw-style multiline composer autosize.