feat(desktop): add a clickable session timeline above the transcript

[HUQIANTAO]

Background

Long conversations lose their shape. Once a session spans many turns and several compactions, there is no way to see where you are in the conversation — how many exchanges happened, where the context got compressed, or which turns are rewind checkpoints. You scroll, and scroll. The transcript already carries all of this structure (user turns, compaction items, rewind checkpointTurn markers), but nothing surfaces it.

What this PR does

Adds a compact, clickable session timeline rail above the transcript:

• Each user turn is a numbered node. The current (latest) turn is filled with the accent color; turns that are rewind checkpoints get a ring.
• Each compaction is a diamond marker sitting on the connecting line, with a tooltip naming the trigger and how many messages were folded.
• Clicking a turn node smooth-scrolls the transcript to that exchange — long sessions become navigable instead of scrollable.
• Hovering a node shows the turn's prompt preview.
• The rail only appears once a session has two or more turns, so short sessions are unaffected.

It is derived entirely from the existing items array (user items + compaction items) and reuses the transcript's own questionAnchorId anchors, so nothing about the transcript or controller changes.

Files changed

• desktop/frontend/src/components/SessionTimeline.tsx — new component
• desktop/frontend/src/App.tsx — render above the transcript
• desktop/frontend/src/styles.css — rail, node, and compaction-marker styles

Verification

• pnpm typecheck — no new errors
• pnpm lint:hooks — clean
• node scripts/check-css-syntax.mjs / check-z-index-tokens.mjs — pass

Documentation-impact: none - a UI navigation aid derived from existing transcript data; no documented behavior changes.