Since the signature became a verbatim, non-editable atom (#476), there
was no way to edit it in the composer at all (#822) - the styling
guarantee silently cost the editability #329 had established. Keep the
atom as the default so branded signatures still survive 1:1, and let an
explicit gesture opt out: double-clicking the block dissolves it into
regular editable content at the same position. Double-clicks that land
on a link are ignored (the click is working the link), and Enter on the
selected block keeps ProseMirror's default create-paragraph-after
behaviour.
Faithfulness of the dissolved signature:
- Leaf <div> lines become paragraphs; only vertical margins the line
does not declare are reset to 0 (how a div actually renders), so
explicit spacing and indents survive. Divs carrying hr/pre and other
non-line blocks are left to the schema parse.
- Inherited styles (font, color, ...) on wrapper divs - which have no
schema node and would be dropped - are hoisted onto the generated
line paragraphs, the line's own declarations winning.
- The content is parsed as a CLOSED document via the schema DOMParser
and spliced in with one transaction; insertContentAt fits an open
slice, which can split text runs at hard breaks and turned <br>-lined
signatures into one spaced paragraph per line.
The bracketing data-signature-block markers live outside the atom and
stay put, so the identity-switch splice and the send path's
already-embedded detection keep working on unlocked content, and
switching identities re-embeds a fresh locked block. Undo restores the
atom. A translated tooltip on the block makes the gesture discoverable.
