Add storage balloon pre-check with actionable error messages because browsers lie about storage space availability.

[nelsonjchen]

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.

If 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 for users who know they can proceed. Holding Shift while skipping exposes the failure UI for testing.

Screenshot

Context

• Discord discussion about the relevance of browser site-storage settings
• Related storage/setup discussion
• Related UX discussion

Validation

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

Implementation note

The existing image-size estimate is not sufficient on its own because browser storage APIs can overstate what the page can actually reserve.

[nelsonjchen]

[nelsonjchen]

Needs a refactor to unslop. too many lines.

[nelsonjchen]

Hints for people testing: Ask your agent to make a sparse file to load some weight. It's a lot faster than making a real file to chew space.

[nelsonjchen]

meh, still a lot of lines so idk. Anyways, if someone wants to follow up with theiir own spin that is smaller, let me know. In the meantime, I'll be passing my fork of the flasher around with this error message until then.

[nelsonjchen]

instances where this helped where a user's hdd was full

https://discord.com/channels/469524606043160576/1540488202127806494/1540519057323720754

[nelsonjchen]

Closing in favor of #233