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.

fix(mobile): Enter inserts newline in channel composer

Confirming this is fixed on iOS: Return now inserts a newline and sending happens from the composer button.

Verified on App Store build 0.11.0. The change landed in #5594 ("make Return insert a newline, send only from the composer button") and is first contained in the mobile-v0.11.0-rc.1 tag.

Looks safe to close.