parseAndValidateKeyring validates that at least one entity in a
fetched keyring matches the pinned root key fingerprint
(rootKeyFingerprint, gpg.go:21). On match, it returns the entire
keyring — including any additional entities the keyserver included
in its response.

This is a problem because openpgp.CheckDetachedSignature iterates
every key in the provided keyring and accepts a signature from any
of them. A compromised or malicious keyserver could return a
response containing the legitimate JetKVM release key (satisfying
the fingerprint check) alongside an attacker-controlled key. A
binary signed with the attacker key would then pass verification
in both VerifySignature and VerifySignatureFromFile, since both
pass the cached keyring directly to CheckDetachedSignature.

The fix is a single-line change: return openpgp.EntityList{entity}
instead of the full keyring when the fingerprint matches. This
ensures only the trusted key is ever used for signature verification
regardless of what a keyserver returns.

TestParseAndValidateKeyring_FiltersRogueKeys exercises this by
constructing a two-entity armored keyring (trusted + rogue),
passing it through parseAndValidateKeyring, asserting the returned
keyring contains exactly one entity with the correct fingerprint,
and confirming that CheckDetachedSignature rejects a signature
produced by the rogue key.

Reported-by: equinox0815

Signed-off-by: Alex Howells <alex@howells.me>

Introduce internal/ota/offline.go with two primary functions:

- ExtractOfflineArchive() reads a .tar.gz upload, validates that it
  contains exactly the expected files (binary + .sha256 + .sig),
  rejects path traversal and unexpected entries, strips leading
  directory prefixes (for archives created with tar czf wrapper dirs),
  and returns an OfflineBundle struct.

- VerifyOfflineBundle() runs SHA256 verification against the hash
  from the archive (hard reject on mismatch), then attempts GPG
  signature verification via the existing GPGVerifier. When keyservers
  are unreachable (air-gapped device), returns KeyFetchFailed=true
  instead of rejecting, allowing the caller to prompt the user for
  bypass confirmation. Bad signatures (key available, sig invalid)
  are always fatal.

Refactor updateSystem() to extract applySystemImage() as a reusable
function for running rk_ota on a staged system tar. Add
ApplyOfflineUpdate() to State for the offline apply flow, plus
GPGVerifier() and ComponentUpdatePath() accessors.

Table-driven tests in offline_test.go cover extraction (valid
app/system archives, missing hash, missing sig, missing binary,
unexpected files, path traversal, corrupt gzip, nested directories)
and verification (valid signature, hash mismatch, invalid signature,
wrong signing key, empty signature, key fetch failure for air-gapped
devices, truncated signature, corrupted binary on disk).

Signed-off-by: Alex Howells <alex@howells.me>

The offline update path previously fell back to a "warn and bypass"
prompt when the device could not reach GPG keyservers to fetch the
signing key. This defeated the purpose of offline updates, which
exist precisely for air-gapped devices without internet access.

The archive format now includes a .pub file (armored GPG public key)
alongside the binary, .sha256, and .sig. On upload, the bundled key
is validated against the pinned root fingerprint in gpg.go:21 via
parseAndValidateKeyring — the same trust anchor used by the online
update path. If the fingerprint matches, the key is used to verify
the signature locally. No keyserver call is made.

Verification is now binary: it passes or it fails. There is no
third "key fetch failed" state and no bypass option.

Backend (internal/ota/offline.go, ota_offline.go):
- OfflineBundle gains PublicKeyData []byte field
- ExtractOfflineArchive requires .pub (4 files, up from 3)
- VerifyOfflineBundle calls new VerifySignatureFromFileWithKey
  method on GPGVerifier instead of VerifySignatureFromFile
- Removed isKeyFetchError(), KeyFetchFailed from OfflineVerifyResult,
  BypassSignature from offlineUpdateApplyRequest, offlineUploadTimeout
- VerifyOfflineBundle no longer takes a context parameter

New GPG method (internal/ota/gpg.go):
- VerifySignatureFromFileWithKey accepts raw armored key bytes,
  validates the fingerprint via parseAndValidateKeyring, then
  calls CheckDetachedSignature with the resulting single-entity
  keyring. No keyserver interaction, no cache mutation.

Frontend (ui/src/components/OfflineUpdateCard.tsx):
- Removed bypass confirmation dialog, showBypassPrompt state,
  keyFetchFailed from UploadResult, bypassSignature from apply
  request, ExclamationTriangleIcon import
- Signature OK indicator now always shows on verified state

Localisation (ui/localization/messages/en.json):
- Removed offline_update_signature_bypass_title,
  offline_update_signature_bypass_description,
  offline_update_signature_bypass_confirm

Tests (internal/ota/offline_test.go):
- All archives now include .pub files
- newOfflineSigningFixture replaces newSigningTestFixture for
  offline tests — no mock HTTP client needed
- Added TestExtractOfflineArchive_MissingPub,
  TestVerifyOfflineBundle_EmptyPublicKey,
  TestVerifyOfflineBundle_WrongKey (bundled key fingerprint
  mismatch)
- Removed TestIsKeyFetchError,
  TestVerifyOfflineBundle_KeyFetchFailure
- End-to-end tests updated for 4-file archive format

Makefile:
- offline_archive_app target includes jetkvm_app.pub

Key rotation is not addressed here. rootKeyFP is a single string;
expanding to []string for the v1→v2→v3→v4 key rollover model is
a separate change.

Signed-off-by: Alex Howells <alex@howells.me>