Web UI: load chat transcript off-DOM with loading skeleton (Phase 2)

[edwin-zvs]

Phase 2 — fixes the blank-flash / jump when opening a chat session.

Problem

loadTranscript revealed an empty transcript pane, then awaited the session.transcript round-trip. During that wait the user saw a blank void (pronounced over the cloudflared tunnel), then content popped in.

Change

• Delayed shimmer skeleton (~150ms) during the fetch instead of a blank pane. Fast local loads resolve before the timer, so there's no skeleton flash.
• Off-DOM build + atomic reveal: the whole transcript is rendered into a detached element (renderEvent retargeted at the builder via activeTranscriptEl), then swapped into the pane and pinned to the bottom in one synchronous step — the user only ever sees the finished transcript already at the latest message; no blank fill, no top-anchored flash that then jumps.
• Session-switch guard: if you click away during the await, the late response no longer clobbers the session you're now viewing.
• Re-selecting an already-loaded session is still instant (cached, no refetch).

Scope / compatibility

Chat-mode only; the terminal pty_replay path is unchanged. No e2e test drives loadTranscript (transcript tests call renderEvent directly, which is untouched), so web_smoke is unaffected. Tail-windowing / lazy-load-older-on-scroll is a deliberate follow-up.

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