feat(app): add a session message navigation rail

[aminghaseminia]

Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a small navigation rail to the session timeline. It shows one short mark per user message along the end edge of the timeline. Hovering the rail reveals the list of user questions, and clicking a mark (or a list entry) scrolls that message into view. On narrow/touch layouts the marks collapse to a slim handle: tapping it opens the list and tapping outside closes it.

I built this because long sessions make it tedious to jump back to an earlier question, and a persistent list of the user's turns makes it fast.

It works by reading the existing visibleUserMessages and reusing the current navigation primitives:

• packages/app/src/pages/session/timeline/message-nav-rail.tsx + .css — the new component. Marks are evenly spaced and centered; the active mark is derived from the scroll position by querying the rendered [data-message-id] rows (the same idea as the existing cursor() in session.tsx). The hover list uses the opaque surface token and truncates each question to one line.
• packages/app/src/pages/session.tsx — mounts the rail over the timeline and jumps through the existing autoScroll.pause() + scrollToMessage() path, so it reuses the same reveal/hash behaviour as the keyboard message navigation.
• packages/app/src/i18n/en.ts — adds session.messages.navRail. Only the English key is added; other locales fall back to English.

Note: CONTRIBUTING says UI features should go through a design review first. I'm happy to pause if this needs that step.

How did you verify your code works?

I could not fully test this. The environment I wrote it in has no Bun and no installed dependencies, so I could not run bun dev, bun typecheck, or the timeline benchmark. What I did verify: both changed TS/TSX files parse under the TypeScript compiler, and the new i18n key is well-formed.

So I have not been able to confirm the runtime behavior or measure the timeline benchmark that packages/app/AGENTS.md asks for before changing timeline code. I'd appreciate it if a maintainer or CI could run those, and I'll iterate on anything that fails.

Screenshots / recordings

Not available yet — the app could not be run in the authoring environment. I can add screenshots/recording once it builds locally.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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