Add Vim keybindings (j/k) for TUI navigation

I propose adding basic Vim motions (specifically j for down and k for up) as an alternative to the standard arrow keys for navigating the model list.

While the current arrow-key navigation works reliably, integrating Vim keybindings is a highly necessary enhancement for a developer-centric CLI tool like this for few reasons: ergonomics, established developer muscle memory, workflow continuity