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/fr.ts

@@ -1309,6 +1309,41 @@ export const catalog: Catalog = {
     "Empty “{name}”?": "Vider « {name} » ?",
     "Delete them": "Les supprimer",
     "Nothing was deleted": "Rien n’a été supprimé",
+    // ── Checking an S/MIME signature, and what may be said about it ──
+    "Certificate covers": "Le certificat couvre",
+    "Details": "Détails",
+    "Earlier messages from this address were signed by {previous}. This one is signed by {current}.": "Les messages précédents de cette adresse étaient signés par {previous}. Celui-ci est signé par {current}.",
+    "Fingerprint": "Empreinte",
+    "Hide details": "Masquer les détails",
+    "Issued by": "Délivré par",
+    "It is signed with OpenPGP, and ihasmail has no way to fetch the sender's public key.": "Il est signé avec OpenPGP, et ihasmail n'a aucun moyen de récupérer la clé publique de l'expéditeur.",
+    "It uses a signature algorithm ihasmail cannot check yet.": "Il utilise un algorithme de signature qu'ihasmail ne sait pas encore vérifier.",
+    "It was made with a certificate belonging to {name}, which does not cover this address.": "Elle a été faite avec un certificat appartenant à {name}, qui ne couvre pas cette adresse.",
+    "Previous fingerprint": "Empreinte précédente",
+    "Signed at": "Signé le",
+    "Signed by {name} — the same signer as before.": "Signé par {name} — le même signataire que précédemment.",
+    "Signed by {name}, seen here for the first time.": "Signé par {name}, vu ici pour la première fois.",
+    "Signer": "Signataire",
+    "That can mean a renewed certificate, and it can mean somebody else. Check with them by some other route before trusting it.": "Cela peut signifier un certificat renouvelé, comme cela peut être quelqu'un d'autre. Vérifiez avec la personne par un autre moyen avant de faire confiance.",
+    "The certificate has expired.": "Le certificat a expiré.",
+    "The certificate is not valid yet.": "Le certificat n'est pas encore valide.",
+    "The message does not match what was signed — it was altered after signing, or damaged on the way.": "Le message ne correspond pas à ce qui a été signé : il a été modifié après signature, ou abîmé en chemin.",
+    "The signature carries no certificate that can be read.": "La signature ne contient aucun certificat lisible.",
+    "The signature could not be read.": "La signature n'a pas pu être lue.",
+    "The signature does not match the certificate sent with it.": "La signature ne correspond pas au certificat envoyé avec elle.",
+    "The signature is not for this sender.": "La signature ne correspond pas à cet expéditeur.",
+    "The signed part is missing either the message or the signature.": "Il manque à la partie signée soit le message, soit la signature.",
+    "The signer has changed.": "Le signataire a changé.",
+    "This message is signed, and ihasmail could not check the signature.": "Ce message est signé, et ihasmail n'a pas pu vérifier la signature.",
+    "This signature does not check out.": "Cette signature ne tient pas.",
+    "Valid until": "Valable jusqu'au",
+    "a different certificate": "un certificat différent",
+    "an unnamed signer": "un signataire sans nom",
+    "as claimed by the signer": "selon le signataire",
+    "first seen {date}": "vu pour la première fois le {date}",
+    "ihasmail will tell you if a later message from this address is signed by anybody else.": "ihasmail vous préviendra si un message ultérieur de cette adresse est signé par quelqu'un d'autre.",
+    "itself, or an issuer it does not name": "lui-même, ou un émetteur qu'il ne nomme pas",
+    "no address": "aucune adresse",
   },
   plurals: {
     // ── Third pass ─────────────────────────────────────────────────────