Feature: share a temporary web preview

## Problem

Inky's **Export for web...** action produces everything needed to play a story in a browser, but sharing a work in progress still requires the writer to understand static hosting and manually upload the exported directory.

This came up in #185: the eventual answer was to upload the generated files to a server or create an itch.io project. That is a reasonable release workflow, but it is a lot of setup when a writer only wants to send a temporary build to a friend, editor, or playtester.

## Proposal

Would you be open to an optional **Share temporary web preview...** action near **Export for web...**?

The intended workflow would be:

1. The writer explicitly chooses **Share temporary web preview...**.
2. Inky builds the same static directory produced by the existing web exporter.
3. After a disclosure/confirmation step, Inky uploads that directory to [temp.md](https://temp.md), a third-party service for temporary static previews.
4. Inky presents the resulting URL with **Open** and **Copy link** actions.
5. Sharing again can update the same canonical URL, so playtesters do not need a new link after every edit.

The current **Export for web...** behavior would remain unchanged. Nothing would be uploaded automatically, and no network request would occur until the writer deliberately chooses the preview action. Before uploading, the UI should clearly explain that the files are being sent to a third party, that the preview is public by default, and that it is temporary.

A failed or cancelled upload would only fail this action; it would not affect the story, local play mode, or normal exports. The integration could reuse `buildForWeb()` and publish its completed output directory, leaving the existing player template and compiler path alone.

temp.md supports anonymous first publishes, multi-file static previews, same-URL updates, and explicit revocation. The service and API implementation are open source at [huangdun/tempmd](https://github.com/huangdun/tempmd), with usage and lifecycle details at [temp.md/docs](https://temp.md/docs).

If this fits Inky's direction, I would be happy to prepare the implementation and accompanying UI text/documentation.

## Alternatives considered

- Keep directing users to itch.io or another static host. This is appropriate for durable releases, but adds account and project setup for quick playtests.
- Send the exported directory or ZIP directly. The recipient still needs to extract it and open it correctly, and updates require sending another copy.
- Add a general deployment system. That seems broader than the temporary sharing problem and would add substantially more configuration and maintenance.

This proposal is only for disposable review and playtest builds, not permanent story hosting or a replacement for the existing export workflow.

Start by tracing the existing Export for web action and the buildForWeb() entry point, then read temp.md's usage and lifecycle documentation. Done means a deliberate preview action can disclose the upload, publish the generated directory, present Open and Copy link actions, support same-URL updates and revocation, and leave normal export behavior unchanged.

Written by the indexing model from the issue text.