Fix --continue picking most recently created conversation instead of most recently used

[ikatyal2110]

When you continue an older conversation with llm -c --conversation <id> and then run llm --continue without an id, it should resume that same conversation. Instead it was picking the most recently created conversation (sorted by conversations.id), not the most recently used one.

The fix queries responses ordered by id desc to find the conversation with the most recent response, matching what llm logs --current already does.

Fixes #1140