Allow configuring Enter key behavior in chat input

[nikomatsakis]

Summary

Currently, pressing Enter in the chat input submits the prompt, and Shift+Enter creates a newline. Some users prefer the opposite: Enter for newline, Shift+Enter (or Cmd+Enter) to submit.

Current behavior

• Enter → submits prompt
• Shift+Enter → inserts newline

Desired behavior (configurable)

Option to swap:

• Enter → inserts newline
• Shift+Enter or Cmd+Enter → submits prompt

Implementation notes

This behavior is controlled by MynahUI (the chat UI library from AWS). Options:

1. Upstream contribution: Fork mynah-ui, implement the configuration option, propose it upstream to aws/mynah-ui
2. Keyboard interception: Inject JavaScript in our webview to intercept and swap the key behaviors

Option 1 is cleaner but requires upstream acceptance. Option 2 is a workaround if needed sooner.

Priority

Nice-to-have, not blocking.