feature request: save preview content to file from interactive mode

Title: feature request: save preview content to file from interactive mode

Use case

When exploring a large JSON document interactively, I often discover a subtree I want to save to a file — but I didn't know I'd need it when I launched fx, so I didn't set up stdout redirection upfront.

This is especially common when a subtree contains a nested JSON string (e.g. an escaped JSON payload inside a string field). I want to save it to a file so I can fx it again to explore the inner structure.

Currently the options are:

• P — prints to stdout and exits fx, requiring fx data.json > output.json to have been set up beforehand
• y then y — copies to clipboard, which works for small values but is awkward for large subtrees (paste into a file externally)

Neither supports a "save and keep exploring" workflow.

Proposal

Add an s key binding in preview mode (p) that prompts for a file path and writes the current node's value to that file, without leaving fx.

Flow:

1. Navigate to the desired node
2. Press p to enter preview mode
3. Press s — a save to: prompt appears
4. Type a file path and press Enter
5. See a confirmation message ("saved to output.json"), continue exploring

The s key doesn't conflict in preview mode since the "show sizes/line numbers" selector only applies in the main view.

I have a working implementation and would be happy to open a PR if this sounds useful.