Improve storage pre-check recovery guidance

[nelsonjchen]

You can try the updated flow at https://comma-flash.mindflakes.com/

Summary

Browsers can report enough available storage while still failing when a large flash image is written to persistent site storage. This adds a storage reservation check as the first step in the existing flashing flow, before device selection.

When the check fails, the UI gives focused recovery guidance:

1. Use a regular browser window rather than Incognito, InPrivate, or Private mode.
2. Free at least 6 GiB of device storage.
3. Review site-data permissions at chrome://settings/content/siteData, including settings that delete data when the browser closes.
4. Fully quit and reopen the browser, then retry.
5. If none of those steps work, try again from an Android phone running Chrome while following the same requirements.

The existing skip action remains available. Holding Shift while clicking Skip storage check exposes the failure UI for testing.

Screenshot

Background

The relevance of the persistent site-data setting was raised in this Discord discussion.

Verification

• pnpm exec vitest run src/utils/image.test.js src/app/App.test.jsx
• pnpm run build