This refactor splits out all conversation concerns into a new
`conversation` package. There is now a split between `conversation` and
`api`s representation of `Message`, the latter storing the minimum
information required for interaction with LLM providers. There is
necessary conversation between the two when making LLM calls.
`tui/tui.go` is no longer responsible for passing window resize updates
to all views, instead we request a new window size message to be sent at
the same time we enter the view, allowing the view to catch and handle
it.
Add `Initialized` to `tui/shared/View` model, now we only call
`Init` on a view before entering it for the first time, rather than
calling `Init` on all views when the application starts.
Renames file, small cleanups
