feat: composer animation for send, streaming and focus

[divyanshub024]

Description

The composer changes state on a single frame: sending, the reply starting to stream, and the field gaining or losing focus all happen with no motion. Sends feel unacknowledged, and a reply in progress is invisible until the first token. Users expect the ChatGPT/Claude like feel.

What you'd like

Animations on FlowComposer, each switchable off in FlowComposerStyle:

• Send: animate the message leaving and the send button acknowledging the tap.
• Streaming: animate the send button into a loading state while isStreaming, and back when the reply ends.
• Focus: animate the composer expanding on focus and collapsing when focus leaves and the field is empty.