Measure signatures in bytes, so the oversize fallback actually saves

Stalwart accepts a signature of `value.len() < 2048`, and that is Rust's len():
2047 bytes of UTF-8. Every check here counted JavaScript `.length` instead,
which is UTF-16 units and agrees only for ASCII — an accent is one unit and two
bytes, CJK three, an emoji two units and four.

That alone would let a non-Latin signature we judged to fit come back rejected.
But the fallback that is supposed to rescue an oversize signature was broken
outright, for everyone: it truncated to `budget - 1` characters and appended an
ellipsis, one character but three bytes, so the result was always 2047
characters and 2049 bytes. Every marker signature Stalwart was ever offered was
two bytes too long, ASCII included. That is why this flow has been sitting in
the README as implemented but unconfirmed — the first person to exceed 2 KB
would have hit it.

Cutting the source text and rendering afterwards, rather than slicing the
rendered string, also means a cut can no longer land inside an HTML entity, and
stepping through code points means it cannot split a surrogate pair.

The old tests used ASCII only, which is how this survived; the new ones weigh
the encoded form.

web/src/views/settings/IdentitiesSettings.tsx

@@ -10,7 +10,7 @@ import { parseAddressList, formatAddressList } from "@/lib/address";
 import { htmlToText } from "@/lib/text";
 import { sanitizeEditorHtml } from "@/lib/html";
 import { externalizeDataImages, storeSignatureHtml, uploadSignatureImage } from "@/lib/signatureImages";
-import { buildMarkerSignature, compactHtml, SIGNATURE_LIMIT } from "@/lib/signatureHtml";
+import { buildMarkerSignature, byteLength, compactHtml, signatureTooLong, SIGNATURE_LIMIT } from "@/lib/signatureHtml";
 
 export function IdentitiesSettings() {
   const identities = useMail((s) => s.identities);
@@ -57,8 +57,9 @@ function IdentityDialog({ identity, onClose }: { identity: Partial<Identity>; on
   const [busy, setBusy] = useState(false);
   const ref = useRef<RichEditorHandle>(null);
   const compact = compactHtml(sanitizeEditorHtml(html));
-  const sigLen = compact.length;
-  const tooLong = sigLen > SIGNATURE_LIMIT || htmlToText(compact).length > SIGNATURE_LIMIT;
+  // The server's limit is on encoded bytes, so that is what to count and show.
+  const sigLen = byteLength(compact);
+  const tooLong = signatureTooLong(compact, htmlToText(compact));
   const save = async () => {
     setBusy(true);
     try {
@@ -67,7 +68,7 @@ function IdentityDialog({ identity, onClose }: { identity: Partial<Identity>; on
       const clean = compactHtml(externalized);
       let htmlSignature = clean;
       let textSignature = htmlToText(clean);
-      if (clean.length > SIGNATURE_LIMIT || textSignature.length > SIGNATURE_LIMIT) {
+      if (signatureTooLong(clean, textSignature)) {
         const blobId = await storeSignatureHtml(clean);
         ({ htmlSignature, textSignature } = buildMarkerSignature(blobId, clean));
       }
@@ -103,7 +104,7 @@ function IdentityDialog({ identity, onClose }: { identity: Partial<Identity>; on
           <span className="hint">Images are stored in your Files (folder “ihasmail”) and embedded when you send.</span>
           <span className="hint nowrap" style={tooLong ? { color: "var(--warn)", fontWeight: 600 } : undefined}>{sigLen.toLocaleString()} / {SIGNATURE_LIMIT.toLocaleString()}</span>
         </div>
-        {tooLong && <div className="warn-box mt-8">This signature is larger than the server's {SIGNATURE_LIMIT}-character limit. ihasmail will keep the full version in your Files and store a short text fallback on the server — other mail clients will see the plain-text version.</div>}
+        {tooLong && <div className="warn-box mt-8">This signature is larger than the server's {SIGNATURE_LIMIT}-byte limit. ihasmail will keep the full version in your Files and store a short text fallback on the server — other mail clients will see the plain-text version.</div>}
       </div>
     </Dialog>
   );