[General] Add a setting to choose whether pressing Enter sends the message or inserts a...

**Mood:** 🎉
**Category:** General

Add a setting to choose whether pressing Enter sends the message or inserts a newline. Two modes:
* Enter to send (current default)
* Shift (or cmd)+Enter to send

This makes composing multiline prompts (code snippets, bullet lists, structured instructions) much less friction-prone, especially for power users writing longer messages.

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| Field | Value |
| --- | --- |
| App version | 0.2.30 |
| OS | macOS 26.5.0 |
| Theme | Fox |
| Path | /chat |
| Tenure | Day 2 (Week 1) |

Start at the /chat experience and trace the current message-composition behavior for Enter. Implement the two requested modes, including Shift or cmd+Enter for sending in multiline mode, and verify that Enter sends or inserts a newline according to the selected setting.

索引モデルが issue の本文から書いたものです。