Add user-message marker rail to conversation scrollbar

[pauldambra]

Problem

In a long conversation it's hard to jump back to a specific thing you asked. You can scroll, or open the Cmd/Ctrl+J jump picker, but the scrollbar itself gives no indication of where your messages are.

Changes

Add a marker rail in the scrollbar gutter of both conversation views (the legacy ConversationView, production default, and the experimental new ChatThread). Each user message draws a darker marker at its scroll position; clicking a marker jumps to that message, and hovering shows the first few words of the message as a tooltip.

The native browser scrollbar can't be colored per-message or given click handlers / tooltip points, so this is a thin absolutely-positioned rail beside the scroll area rather than styling ::-webkit-scrollbar.

• New scrollbar-rail/ module: MessageScrollbarRail (presentation) + useMessageRailMarkers (measures rendered user-message rows and computes fractional positions; interpolates unmeasured/virtualized rows between measured neighbours).
• Legacy ConversationView: VirtualizedList exposes its scroll + content elements via the imperative handle; the rail mounts in the container over the scrollbar gutter.
• New ChatThread: a ThreadScrollbarRail locates the quill scroller viewport + content via a hidden probe (same pattern as StickyHeaderOverlay) and mounts the rail inside ChatMessageScroller.

How did I test this?

• Added unit tests for the label helper and the rail component (MessageScrollbarRail.test.tsx): truncation, marker count, fractional positioning, and click → jump. All 8 pass.
• pnpm exec biome lint + format clean on touched files.
• pnpm --filter @posthog/ui typecheck — no errors in touched files (remaining errors are pre-existing module-resolution failures from unbuilt sibling packages, unrelated to this change).

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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[github-actions]

React Doctor found 3 issues in 2 files · 3 warnings.

3 warnings

src/features/sessions/components/ConversationView.tsx

• ⚠️ src/features/sessions/components/ConversationView.tsx:123 Many related useState calls prefer-useReducer
• ⚠️ src/features/sessions/components/ConversationView.tsx:360 State initialized from a mount effect no-initialize-state

src/features/sessions/components/scrollbar-rail/MessageScrollbarRail.tsx

• ⚠️ src/features/sessions/components/scrollbar-rail/MessageScrollbarRail.tsx:53 Control missing accessible label control-has-associated-label

_{Reviewed by React Doctor for commit 79ed5bb.}

[posthog]

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