Mobile channel composer: make Enter insert a newline instead of sending

**Motivation**
In the Buzz mobile channel composer, the keyboard Enter/Return action sends the message. This makes multiline messages difficult to write, even though the composer supports up to five lines. The visible send button already provides a clear way to send.

**Proposed solution**
Make Enter/Return insert a newline in the mobile channel composer and keep the visible send button as the send action. Optionally add a preference for users who want Enter-to-send, with newline as the default.

**Alternatives considered**
Keep Enter-to-send and add a separate newline affordance. This is less consistent with multiline mobile text input and adds UI complexity.

**Additional context**
The current channel composer uses `TextInputAction.send` and calls `onSend()` from `onSubmitted` in `mobile/lib/features/channels/compose_bar/layout.dart`. No matching open or closed issue was found.