fix: return no-JS form submissions to the submitting page when an action returns json()

[birkskyum]

• Closes [Bug?]: return json() in action is not SSR ready #1976

Without JavaScript, a form posting to an action that returns json() or reload() got a 302 with no Location header, leaving the browser parked on /_server?id=.... These responses carry a value rather than a destination, so handleNoJS never set a redirect target.

• Fall back to the referer when a returned Response has no Location
• Unwrap customBody() into the flash cookie so useSubmission().result matches the JS path (it previously serialized the Response to {})
• Fall back to the app root instead of throwing when there is no usable referer, and guard the flash input against an empty argument list

Adds a no-JS e2e suite covering json(), reload(), redirect(), and a plain return. The first two fail on main and pass here; the latter two were already correct.

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Latest commit: 47e581d

The changes in this PR will be included in the next version bump.

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@solidjs/start	Patch

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