fix(ui): close picker on empty items

[hermandavid]

Summary

• When a reload action (e.g. <C-d> delete in the session picker) results in an empty item list, the picker now closes automatically instead of staying open in an invalid empty state.
• Adds a close function to PickerOptions that each backend sets with its own cleanup logic, and checks for empty results in all reload handlers.

Problem

Deleting the last session with <C-d> left the picker open with no items. There was no handling for the empty-list case after a reload action resolved, so the picker would refresh with zero entries.

With an empty picker, no recovery was possible either -- all action handlers in every backend (Telescope, fzf-lua, mini.pick, Snacks) guard execution on having a currently selected item, so <C-s> (new session) would silently do nothing. The user's only option was to dismiss the broken picker and reopen it.

Approach

Each picker backend now provides an opts.close() implementation using its native close mechanism:

• Telescope: actions.close() with selection_made guard
• fzf-lua: closes the fzf window via  fzf-lua.win with a closed flag to prevent double-callback from default/esc handlers
• mini.pick: mini_pick.stop(), with callback handled naturally by on_done
• Snacks: _picker:close() with selection_made guard

All reload handlers check #resolved_items == 0 and call opts.close() instead of refreshing.

[sudo-tee]

Thanks for the PR
It looks good to me