feat(investigations): Show viewers on the investigation page

[wedamija]

Show who else has opened an investigation next to "Last update": people viewing it now first, with a green ring, then earlier viewers, faded. The list comes from the presence heartbeat, at the interval the backend returns.

[github-actions]

Story previews

Preview the stories changed in this PR on the Vercel deployment:

• app/views/investigations/detail/index.stories.tsx

_{Preview deployment: https://sentry-7or0dspvs.sentry.dev}

[sentry]

Bug: The viewer overflow count +N is calculated using viewers.length instead of shown.length, leading to an inaccurate count when some viewers are no longer organization members.
_{Severity: LOW}

Suggested Fix

The overflow count calculation should be changed from total - viewers.length to total - shown.length to accurately reflect the number of hidden viewers.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: static/app/views/investigations/detail/presence.tsx#L86-L88

Potential issue: The overflow count for viewers is calculated as `total -
viewers.length`. However, the number of avatars actually rendered is based on
`shown.length`. If a viewer has left the organization, their `userId` won't resolve to a
member, causing `shown.length` to be less than `viewers.length`. This results in the
`+N` overflow count underestimating the number of hidden viewers, making the total count
implied by the UI incorrect.

_{Did we get this right? 👍 / 👎 to inform future reviews.}