Check S/MIME signatures, and remember who signed

A signed message now says whether that holds up, as it is read. This is
verification only: nothing here signs, encrypts or decrypts, and the
private-key question that blocks those is untouched. Verifying needed
none of it, because the certificate travels inside the message -- which
is why this is the half that could be built.

What it checks. For multipart/signed carrying PKCS#7, the exact bytes of
the signed part -- headers included, canonicalised to CRLF -- are hashed
against the messageDigest attribute, and the signature over the signed
attributes is verified with WebCrypto against the certificate inside the
message. RSA PKCS#1 v1.5 and ECDSA over P-256/384/521, with SHA-256, 384
or 512.

The trust model is the design, and it is deliberately small. A browser
has no system trust store, and the certificate arrives inside the
message, so anyone can self-sign as anyone: on its own a good signature
shows only that the sender held the key they attached. So the word
"verified" is never rendered, and the reassuring case is not the loud
one. What carries the weight is remembering -- the first signed message
from an address pins its fingerprint, later ones are compared, and a
signer that changed is reported with both names and told to check by
another route. Trust on first use, no certificate authority anywhere.

The pins live in the account's settings rather than the browser: one
that only a single device knew would greet the same correspondent as new
everywhere else, which is how people are trained to click past the one
warning that matters. A pin records the message that created it, so the
message that established a signer keeps saying so instead of appearing
to be corroborated by itself -- without that, the very first signed
message anybody receives reads as "the same signer as before", where
before is itself. A changed, mismatched or expired signer is never
pinned, since writing the anomaly into the baseline makes every later
message agree with it.

Three things are declined rather than attempted, and all three say
"could not check" rather than "does not check out", because ignorance
and an accusation are different claims:

  - OpenPGP, by name. The signature carries no key and there is nowhere
    to get the sender's: x:PublicKey is the account's OWN registry, and
    a keyserver or WKD lookup would tell a third party who you
    correspond with -- the leak the image proxy exists to close.
  - SHA-1. Not forgeable in practice today, still not something to put a
    tick beside.
  - RSA-PSS, whose salt length lives in parameters this does not read.
    Guessing wrong would report a good signature as bad.

Nothing validates a chain: no CA bundle is shipped and revocation is not
checked. "Issued by" reports what the certificate claims, and a
self-signed one claims itself.

The DER, CMS, X.509 and MIME readers are hand-written and deliberately
narrow -- no new dependency, and the whole verifier is a lazily imported
8.6 kB chunk that a reader of unsigned mail never downloads. The one
place this is easy to get quietly wrong has its own function and its own
test: signed attributes are signed as a SET OF, not as the [0] IMPLICIT
they arrive as, and hashing the message instead would make every
signature "pass".

Tested against real `openssl smime -sign` output rather than hand-built
fixtures -- RSA, ECDSA, a tampered copy, and a valid signature by a
certificate for somebody else -- because a signed message written by
hand only agrees with whatever its author believed the format to be.
Also driven in a browser against the mock, which now serves three real
signed messages so every branch of the banner is reachable.

Translations: 34 new strings in all nine catalogues, 306 entries.
Falling back to English is unchanged at 24 per language.

web/src/locales/es.ts

@@ -1304,6 +1304,41 @@ export const catalog: Catalog = {
     "Empty “{name}”?": "¿Vaciar «{name}»?",
     "Delete them": "Eliminarlos",
     "Nothing was deleted": "No se ha eliminado nada",
+    // ── Checking an S/MIME signature, and what may be said about it ──
+    "Certificate covers": "El certificado cubre",
+    "Details": "Detalles",
+    "Earlier messages from this address were signed by {previous}. This one is signed by {current}.": "Los mensajes anteriores de esta dirección los firmaba {previous}. Este lo firma {current}.",
+    "Fingerprint": "Huella digital",
+    "Hide details": "Ocultar detalles",
+    "Issued by": "Emitido por",
+    "It is signed with OpenPGP, and ihasmail has no way to fetch the sender's public key.": "Está firmado con OpenPGP, y ihasmail no tiene forma de obtener la clave pública del remitente.",
+    "It uses a signature algorithm ihasmail cannot check yet.": "Usa un algoritmo de firma que ihasmail todavía no puede comprobar.",
+    "It was made with a certificate belonging to {name}, which does not cover this address.": "Se hizo con un certificado de {name}, que no cubre esta dirección.",
+    "Previous fingerprint": "Huella digital anterior",
+    "Signed at": "Firmado el",
+    "Signed by {name} — the same signer as before.": "Firmado por {name}: el mismo firmante que antes.",
+    "Signed by {name}, seen here for the first time.": "Firmado por {name}, visto aquí por primera vez.",
+    "Signer": "Firmante",
+    "That can mean a renewed certificate, and it can mean somebody else. Check with them by some other route before trusting it.": "Puede tratarse de un certificado renovado, y puede tratarse de otra persona. Compruébelo con ella por otra vía antes de confiar.",
+    "The certificate has expired.": "El certificado ha caducado.",
+    "The certificate is not valid yet.": "El certificado aún no es válido.",
+    "The message does not match what was signed — it was altered after signing, or damaged on the way.": "El mensaje no coincide con lo que se firmó: se alteró después de firmarlo, o se dañó por el camino.",
+    "The signature carries no certificate that can be read.": "La firma no lleva ningún certificado que se pueda leer.",
+    "The signature could not be read.": "No se ha podido leer la firma.",
+    "The signature does not match the certificate sent with it.": "La firma no coincide con el certificado enviado con ella.",
+    "The signature is not for this sender.": "La firma no corresponde a este remitente.",
+    "The signed part is missing either the message or the signature.": "A la parte firmada le falta el mensaje o la firma.",
+    "The signer has changed.": "El firmante ha cambiado.",
+    "This message is signed, and ihasmail could not check the signature.": "Este mensaje está firmado, y ihasmail no ha podido comprobar la firma.",
+    "This signature does not check out.": "Esta firma no cuadra.",
+    "Valid until": "Válido hasta",
+    "a different certificate": "un certificado distinto",
+    "an unnamed signer": "un firmante sin nombre",
+    "as claimed by the signer": "según declara el firmante",
+    "first seen {date}": "visto por primera vez el {date}",
+    "ihasmail will tell you if a later message from this address is signed by anybody else.": "ihasmail le avisará si un mensaje posterior de esta dirección lo firma otra persona.",
+    "itself, or an issuer it does not name": "sí mismo, o un emisor que no nombra",
+    "no address": "ninguna dirección",
   },
   plurals: {
     // ── Third pass ─────────────────────────────────────────────────────