feat: add config option for sidebar default state

[marcelarie]

Adds a new tui.sidebar config option to set the sidebar's initial state. Accepts "auto" (default), "show", or "hide".

• Config provides initial default before user interaction
• User toggles via keybind are stored in KV and take precedence
• Priority: KV store > config.tui.sidebar > "auto"

Closes #3682