Adds a transcript surface for a Structured-view session alongside the
existing xterm attach view: a WS /api/structured/:id connection
(structured.ts) mounted by ConnectionPool whenever a session's `view`
is `"Structured"`, and pure per-row/rail renderers (structuredRender.ts,
structuredDiff.ts) covering all eight transcript row kinds.
Hierarchy follows the design reference (artifact 88ecd690): the
approval is the only lifted, card-like element; a user turn is a
tinted block with an accent rail; agent prose is plain text; tool
calls are compact bordered rows expanding to output or a coloured,
horizontally-scrolling diff computed from ACP's full old/new text.
The client renders `reduced_state` and rows exactly as received, with
no raw-event folding or turn tracking of its own.
Extracted attach.ts's pane-header title building into paneTitle.ts and
its connection interface into pane.ts (PaneConnection) so both
connection kinds share one contract and one header.
Miguel Palhas