Layout cleanups: smaller header and single line message input

[toolmantim]

Updates the overall chat layout to be waste less vertical space, and improves the chat input context UI:

• Smaller header (no more logo)
• Single line message compose box
• Replace short version of codebase name from chat input context info, in favour of a better title on the repository icon that has the full repository
• Chat input context shows "No file selected" when no files are open

Things that I didn't manage to fix, but would love to get some eng help on follow-ups:

• Height of input should change when pasting long paragraphs, not just use the newline count to change [rows] ala https://gomakethings.com/autogrowing-textareas/ or similar
• Editing a message, which uses it's own separate textarea
• Removing the tabs: waiting on a quickpick to replace recipes (then I can add a button up the top, or button in the compose box)

Before	After

Test plan

• Open Cody chat
• Manual interaction

[beyang]

Looks great!