snacks.nvim leader-space file picker is user config, not a default

When a user asks why the Neovim file finder is on <leader><space>, check their own snacks.nvim keymaps first: snacks.picker has no default leader maps, so the binding comes from the user's config, usually copied from the LazyVim/Doom convention. To move it, rebind the snacks.picker.files() map to <leader>ff and delete the old key instead of adding a second map. Before you do, grep for other <leader>f* maps so the new chord does not shadow an existing prefix.