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fix(web): stretch the flash key from the deployment secret (#3361)

* fix(web): stretch the flash key from the deployment secret

A captured flash cookie is an offline oracle for `secret`, and the option
accepts any non-empty string. Under a single SHA-256 each guess cost one
hash, so a short or human-chosen secret fell quickly. Recovering it means
reading every flash payload and forging new ones.

- Derive the key with PBKDF2-HMAC-SHA-256 at 100,000 iterations, salted with
  the same domain string as before. The cap is the ceiling some edge runtimes
  enforce on PBKDF2.
- Document the entropy requirement on the `secret` option and show how to
  generate a value. Stretching raises the price of a guess. It does not make
  a guessable secret safe.

The derivation runs once per process per secret and only on the no-JS form
path, so ordinary requests are unaffected. The derived key changes, so
in-flight flashes read as no flash, the same as a secret rotation.

Tested with a spec that attacks the cookie the public API produces: the old
bare digest must not open it, PBKDF2 with the documented parameters must, and
neither a lower iteration count nor another domain salt does.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

* test(web): type the packed flash bytes as Uint8Array<ArrayBuffer>

`subarray` on a plain `Uint8Array` parameter is `Uint8Array<ArrayBufferLike>`,
which `crypto.subtle.decrypt` rejects as a `BufferSource` under the current
lib types. The bytes come from `new Uint8Array(n)`, so the narrower type
is the true one.

Co-authored-by: Claude via Cursor <noreply@cursor.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Ryan Carniato <ryansolid@gmail.com>
Co-authored-by: Claude via Cursor <noreply@cursor.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

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