fix(migration): authenticate 0x03 framing and harden staged-file lifecycle

Address Greptile review feedback on the chunked export format:

- Authenticate 0x03 framing so the stream cannot be reordered, replayed,
  or truncated. Each chunk IV is now derived from (base IV, index) on the
  decrypt side instead of being read from the file, and each chunk's GCM
  AAD binds the fixed header plus the chunk index into the tag. After the
  final chunk, the stream verifies the file is exhausted, so a lowered
  total length (authenticated-prefix attack) is rejected.
- Apply the 8 MiB chunk cap while the length is still a Long so exports
  between 2 and 4 GiB no longer wrap to a zero chunk size and divide by
  zero.
- Reject a zero chunk size for non-empty plaintext.
- Delete the staged bundle file on every readMigrationBundle failure path
  (wrong password, password-required, decrypt/manifest decode) so repeated
  failed attempts do not leak full bundle copies or a plaintext ZIP.
- Track the in-flight preview job and guard the install with a sequence
  counter so a slow, superseded preview cannot overwrite the newer
  preview's staged file or UI state; the replaced staged file is deleted.
- Add regression tests for framing tamper, ciphertext tamper, trailing
  data, negative total length, zero-chunk header, and a multi-chunk
  round trip.

Author: torlando-tech