Components.AI: Implement composer keyboard, IME, and autogrow behavior

### Is your feature request related to a problem? Please describe the problem.

Enter always submits, Shift+Enter cannot insert a newline, IME composition can submit prematurely, and the textarea does not grow with content.

### Describe the solution you'd like

Implement configurable submit keybindings with Enter-to-send and Shift+Enter newline by default, guard IME composition, and use CSS `field-sizing: content` with a max-height and internal overflow. Reset sizing after send.

### Additional context

Leaf task for #68227 requirements C5, H1, and H2.

Start by locating the Components.AI composer implementation and its textarea, send handling, and styling. Verify Enter-to-send, Shift+Enter newlines, IME composition protection, CSS content sizing with a maximum height and internal overflow, and sizing reset after sending; use any nearby tests if present.

Written by the indexing model from the issue text.