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.
Improves render handling by moving the responsibility of laying out the
whole UI from each view and into the main `tui` model. Our `ViewModel`
interface has now diverged from bubbletea's `Model` and introduces
individual `Header`, `Content`, and `Footer` methods for rendering those
UI elements.
Also moved away from using value receivers on our Update and View
functions (as is common across Bubbletea) to pointer receivers, which
cleaned up some of the weirder aspects of the code (e.g. before we
essentially had no choice but to do our rendering in `Update` in order
to calculate and update the final height of the main content's
`viewport`).
`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
When the last message in the passed messages slice is an assistant
message, treat it as a partial message that is being continued, and
include its content in the newly created reply
Update TUI code to handle new behavior
Better handling of persistence, and we now ensure the response we
persist is trimmed of whitespace, particularly important when a response
is cancelled mid-stream
