Compose-box layout redesign

[gnprice]

From @terpimost we have a spiffy new design for the compose box:

Details are in Figma:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=3954-13395&m=dev

This issue is for updating the compose box to match that design. That means:

• the white background, the border from the message list, where applicable the separator between topic and content
• the text styling for topic and for content
• the styling of the send button and the compose buttons (attach file, take photo, etc.)
• the height and scrolling behavior of the content input
• other related details I'm overlooking at the moment

Out of scope:

• the set of buttons that exist — adding buttons corresponds to new features, which will happen in their own issues
• the "..." overflow button, scrolling, and fading for excess compose buttons — for now we have just three of them, not enough to make those come into effect
• the autocomplete results — those are Redesign for @-mention autocomplete items #913 and Redesign for autocomplete result lists #914
• editing a message — that's compose: Support editing an already-sent message #126
• loading state and error feedback for message send — those are tracked in Await send request; handle failure #720