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/nl.ts

@@ -1300,6 +1300,41 @@ export const catalog: Catalog = {
     "Empty “{name}”?": "„{name}” leegmaken?",
     "Delete them": "Verwijderen",
     "Nothing was deleted": "Er is niets verwijderd",
+    // ── Checking an S/MIME signature, and what may be said about it ──
+    "Certificate covers": "Certificaat geldt voor",
+    "Details": "Details",
+    "Earlier messages from this address were signed by {previous}. This one is signed by {current}.": "Eerdere berichten van dit adres waren ondertekend door {previous}. Dit bericht is ondertekend door {current}.",
+    "Fingerprint": "Vingerafdruk",
+    "Hide details": "Details verbergen",
+    "Issued by": "Uitgegeven door",
+    "It is signed with OpenPGP, and ihasmail has no way to fetch the sender's public key.": "Het is ondertekend met OpenPGP, en ihasmail kan de openbare sleutel van de afzender niet ophalen.",
+    "It uses a signature algorithm ihasmail cannot check yet.": "Het gebruikt een ondertekeningsalgoritme dat ihasmail nog niet kan controleren.",
+    "It was made with a certificate belonging to {name}, which does not cover this address.": "Hij is gemaakt met een certificaat van {name}, dat dit adres niet dekt.",
+    "Previous fingerprint": "Vorige vingerafdruk",
+    "Signed at": "Ondertekend op",
+    "Signed by {name} — the same signer as before.": "Ondertekend door {name} — dezelfde ondertekenaar als eerder.",
+    "Signed by {name}, seen here for the first time.": "Ondertekend door {name}, hier voor het eerst gezien.",
+    "Signer": "Ondertekenaar",
+    "That can mean a renewed certificate, and it can mean somebody else. Check with them by some other route before trusting it.": "Dat kan een vernieuwd certificaat betekenen, en het kan iemand anders zijn. Vraag het langs een andere weg na voordat u erop vertrouwt.",
+    "The certificate has expired.": "Het certificaat is verlopen.",
+    "The certificate is not valid yet.": "Het certificaat is nog niet geldig.",
+    "The message does not match what was signed — it was altered after signing, or damaged on the way.": "Het bericht komt niet overeen met wat er is ondertekend — het is na ondertekening gewijzigd, of onderweg beschadigd.",
+    "The signature carries no certificate that can be read.": "De handtekening bevat geen leesbaar certificaat.",
+    "The signature could not be read.": "De handtekening kon niet worden gelezen.",
+    "The signature does not match the certificate sent with it.": "De handtekening hoort niet bij het meegestuurde certificaat.",
+    "The signature is not for this sender.": "De handtekening is niet van deze afzender.",
+    "The signed part is missing either the message or the signature.": "In het ondertekende deel ontbreekt het bericht of de handtekening.",
+    "The signer has changed.": "De ondertekenaar is veranderd.",
+    "This message is signed, and ihasmail could not check the signature.": "Dit bericht is ondertekend, en ihasmail kon de handtekening niet controleren.",
+    "This signature does not check out.": "Deze handtekening klopt niet.",
+    "Valid until": "Geldig tot",
+    "a different certificate": "een ander certificaat",
+    "an unnamed signer": "een naamloze ondertekenaar",
+    "as claimed by the signer": "volgens de ondertekenaar",
+    "first seen {date}": "voor het eerst gezien op {date}",
+    "ihasmail will tell you if a later message from this address is signed by anybody else.": "ihasmail laat het weten als een later bericht van dit adres door iemand anders is ondertekend.",
+    "itself, or an issuer it does not name": "zichzelf, of een uitgever die het niet noemt",
+    "no address": "geen adres",
   },
   plurals: {
     // ── Third pass ─────────────────────────────────────────────────────