[Feature Request] Option to use Ctrl+Enter to submit instead of Enter

Problem

When typing multi-line messages, accidentally pressing Enter submits the message immediately. This interrupts the workflow and causes incomplete messages to be sent.

Proposed Solution

Add a setting to change the submit key behavior:

{
  "claude-code.submitKey": "ctrl+enter"  // or "enter" (default)
}

Behavior options:

• enter (default): Enter submits, Shift+Enter for newline
• ctrl+enter: Enter for newline, Ctrl+Enter submits

Prior Art

Most chat applications support this pattern:

• Google Chat: Ctrl+Enter to send
• Slack: Option in settings
• Discord: Shift+Enter for newline
• Telegram Desktop: Ctrl+Enter option

Use Case

Power users who write detailed, multi-line prompts frequently hit Enter by habit and accidentally submit incomplete messages. This is especially frustrating in VSCode extension where the chat input feels like a code editor.

Implementation

This should be a simple keybinding configuration change in the VSCode extension.