feature: option for the floating window's keymaps

### Did you check the docs?

- [X] I have read the Grapple docs

### Is your feature request related to a problem? Please describe.

I want to rebind `Select (split)` and `Select (vsplit)`.

### Describe the solution you'd like

> ```
> require("oil").setup({
> ...,
> keymaps = {
> ["g?"] = "actions.show_help",
> [""] = "actions.select",
> [""] = { "actions.select", opts = { vertical = true }, desc = "..." },
> ...
> },
> ...
> })
> ```

Something similar to [oil.nvim](https://github.com/stevearc/oil.nvim?tab=readme-ov-file#options).

### Describe alternatives you've considered

None.

### Additional context

_No response_

Start by tracing how the plugin setup handles keymaps and how the floating window defines the Select (split) and Select (vsplit) mappings. Compare the requested configuration with the existing setup and confirm that both mappings can be rebound with per-action options; done when the configured mappings control those actions.

Written by the indexing model from the issue text.