- Pass around message/conversation values instead of pointers where it
makes more sense, and store values instead of pointers in the globally
(within the TUI) shared `App` (pointers provide no utility here).
- Split conversation persistence into separate conversation/message
saving stages
Replaced `LatestConversationMessages` with `LoadConversationList`, which
utilizes `LastMessageAt` for much faster conversation loading in the
conversation listing TUI and `lmcli list` command.
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.
