Mobile: activity ("code") cards default to collapsed while running

When you send a message on mobile, the agent's activity card auto-expands while the run is live: the card balloons as code streams in, then collapses back to a one-line summary when the run settles. Jarring both ways — the chat jumps huge, then the content vanishes.

The collapsed summary row already carries everything a live run needs — the spinner (ActivityIndicator while status === "running"), the live summary line ("writing code…", "running code…"), and the approval glyphs — so the card can stay a minimally-jarring little line unless the user taps it open.

Current behavior lives in apps/mobile/src/components/activity-card.tsx:

const [toggled, setToggled] = useState<boolean | null>(null);
// Live activities stream open so you can watch the code being written;
// settled ones collapse to their summary until tapped.
const expanded = toggled ?? isLive;

Checklist#

ActivityCard defaults to collapsed regardless of live status — expanded only by tap (toggled === true) — activity-card.tsx, one-line change plus comments
Keep RoundView's auto-expand of a running round — it only renders inside a card the user deliberately opened, where watching the live round is the point — untouched
Update the component doc comment (no more "automatically while live-streaming") — done
Verify the chat-titles spec still passes (it rides the collapsed card's spinner while its /script runs) — passes locally, including in video mode
VIDEO_MODE recording for the PR body showing the calm collapsed line — in the PR body
typecheck / lint / knip / format / test green — all green locally

Non-goals#

The os web feed's expansion behavior (apps/os) — mobile only.
Changing what the expanded card renders (rounds/tabs stay as they are).