feat(investigations): Add a read-only Activity section

[wedamija]

We're adding comments to Investigations, and this is one of a series of frontend PRs that build the feature. Comments show in an Activity section at the bottom of the page, and block comments open in a thread drawer. Everything is behind the investigations flag.

This PR adds the Activity section, which only displays comments for now:

• It uses the issue activity feed's timeline layout.
• Each comment shows as "Name commented (on ) • time", with the avatar on the timeline, newest first.
• The last row is "Investigation first created".
• It shows the latest 100 comments, with a note when there are older ones.

Later PRs in this stack add posting comments, then edit and delete.

[github-actions]

Story previews

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

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

_{Preview deployment: https://sentry-c2nsosdpd.sentry.dev}

[billyvg]

🤖 AI review by Claude, an AI agent. I'm reviewing this stack at @billyvg's request and wrote the inline comments below. @wedamija, Billy will also go through these comments and the code, so please treat this as a first pass, not a final review.

This merged while I was reviewing, so the two nits below are optional follow-ups, not blockers.

Overall this looks good. Reusing the issue activity timeline pieces keeps the feed consistent with issues, and moving getBlockDisplayTitle out of the cell is a good way to share it. I only have two small, optional nits.

[billyvg]

Visual QA

Screenshots from Scraps stories (fixture data, 1440×900, light theme).

Story coverage: the stories' fixture API (__stories__/investigationFixtureApi.tsx) throws on any investigation path it doesn't know, including /comments/. So on this branch, 9 of the existing Detail stories show "There was an error loading data" in the new Activity section. I added comment handling to the fixture (none by default), Activity stories, and a full-page comments story, in a local commit that isn't pushed yet.

before (base 092b744)	this branch as-is	with the fixture fix

New stories

Page and block comments	More than 100 comments

On a full investigation page:

Newest first, "on <block>", "on a deleted block" and "Unknown user" all render as described.

Nit: "Showing the latest 100 comments." sits 3px above the centre of its timeline marker. The text is a bare 16px Text, while comment rows wrap theirs in a Flex with minHeight="22px" and align="center"; the same wrapper would line it up.