fix(ui): show Loading rather than "No messages yet" while history loads

[mprokopov]

A populated session renders as empty for as long as its history takes to load.

For a runtime-backed session the chat pane derives messages from the runtime event history rather than the persisted message store (page.tsx, displayMessages). runtimeEventsToMessages returns an array, so before the replay resolves it returns []:

• the empty state renders on displayMessages.length === 0
• Loading… renders only on !displayMessages

So the pane shows "No messages yet. Type a message below to start the conversation." for the entire replay, on a session that has plenty of messages.

What it looks like

A 38-event session took 20s to replay (see #489 for why). For those 20s the UI actively states the session is empty and invites you to start it — which reads as data loss, not as loading. The events were all present the whole time: 18 agent.message among them.

Change

Hold runtimeMessages at null until the replay has returned, so the existing loading branch is used. Three details:

• the flag resets per session, alongside the other per-session state, so switching sessions cannot show the previous one's result
• it is set on failure as well as success. listRuntimeEvents is deliberately best-effort — it returns [] for a non-JSON or error response rather than throwing — so a failed replay has to fall through to the empty state instead of sitting on Loading… forever
• dependency arrays updated; eslint reports no new exhaustive-deps warning

Checks

tsc --noEmit exit 0, eslint exit 0 (one pre-existing warning in onboarding/page.tsx, unrelated).

Relationship to #489

Independent but complementary. #489 makes the replay fast; this makes the wait honest. Either alone leaves a real gap — with #489 only, a slow or large session still flashes a false empty state; with this only, you wait 20s on a spinner.