fix(updater): accept tauri base64-wrapped minisign signatures

`tauri signer sign` emits .sig files that are base64-wrapped minisign
documents — the same wrapping already handled for the pinned public key —
but verifySignatureReader fed the wrapped text straight to
minisign.Signature.UnmarshalText, so every real release signature failed to
parse ("minisign: invalid signature"). Unwrap base64 when the text is not a
raw minisign document; raw documents (and the test fixtures) pass through
unchanged.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Server/updater/updater.go

@@ -555,7 +555,7 @@ func (u *Updater) verifySignatureReader(reader io.Reader, signatureText []byte,
 		return fmt.Errorf("reading file for signature verification: %w", err)
 	}
 
-	normalizedSig := []byte(strings.TrimSpace(string(signatureText)))
+	normalizedSig := normalizeSignatureText(signatureText)
 	var parsedSig minisign.Signature
 	if err := parsedSig.UnmarshalText(normalizedSig); err != nil {
 		return fmt.Errorf("invalid update signature format: %w", err)
@@ -567,6 +567,21 @@ func (u *Updater) verifySignatureReader(reader io.Reader, signatureText []byte,
 	return nil
 }
 
+// normalizeSignatureText returns the raw minisign signature document from
+// signatureText. `tauri signer sign` emits .sig files that are base64-wrapped
+// minisign documents (the same wrapping used for the pinned public key file);
+// raw minisign documents pass through unchanged.
+func normalizeSignatureText(signatureText []byte) []byte {
+	trimmed := []byte(strings.TrimSpace(string(signatureText)))
+	if bytes.HasPrefix(trimmed, []byte("untrusted comment:")) {
+		return trimmed
+	}
+	if decoded, err := base64.StdEncoding.DecodeString(string(trimmed)); err == nil {
+		return []byte(strings.TrimSpace(string(decoded)))
+	}
+	return trimmed
+}
+
 func (u *Updater) serverSignaturePublicKey() (minisign.PublicKey, error) {
 	decoded, err := base64.StdEncoding.DecodeString(u.signingKeyText)
 	if err != nil {