Signatures render into the main document - the identity form's live preview
and the composer's signature block - rather than the sandboxed iframe used for
message bodies. SIGNATURE_SANITIZE_CONFIG allows no target attribute, so those
anchors were live and target-less: one click navigated the whole app away,
discarding the unsent draft or the unsaved signature with it.
Add sanitizeSignatureHtmlForDisplay, which keeps the storage sanitizer's image
restrictions but forces target="_blank" rel="noopener noreferrer" on every
anchor, and use it at the two render sites. The composer's SignatureBlock
NodeView stamps the target on its rendered DOM instead, because attrs.html is
what serializeEditorContent emits into the sent message - storage and the
recipient's copy stay exactly as the user wrote them.
