fix(chat): rework prompt navigator rail into sliding tape with hover preview (openchamber#2185)

* fix(chat): rework prompt navigator rail into sliding tape with hover preview

- Pin the active indicator to the target during programmatic scrolls so the
  scroll spy's intermediate reports don't drag it backwards mid-animation
- Replace the visibility-ratio active-turn picker with a stable reading-line
  rule (last turn whose top is above the line), dropping IntersectionObserver
- Replace the list panel with a Codex-style gutter: the whole strip is one
  hover/click target mapped to the nearest tick, with a per-prompt preview
  card that follows the cursor
- Cap the rail at a fixed window of ticks; hovering the edges carousels
  through the rest, with gradient masks hinting at more content
- Render ticks as a tape that glides to keep the active prompt centered,
  remounting on history prepend to avoid spurious slide animations
- Keep load-earlier as a compact button aligned over the tick column

* fix(chat): shrink navigator gutter when message column sits under it

On narrow windows the centered message column extends under the rail's
full-width invisible hover zone, which swallowed clicks on the right edge
of user bubbles — including the expand/collapse control. Measure the
column against the gutter and switch to a narrow hit zone when they
overlap.

* fix(sync): stop runaway history auto-load on sessions with empty assistant messages

An assistant message fetched with zero parts (e.g. a run aborted before any
output) was stored as absence — indistinguishable from parts that were never
fetched. getSessionMaterializationStatus therefore reported the session as
never renderable, so the ensure-renderable effects (ChatContainer,
ModelControls) retried syncSession forever; each retry refetched the whole
grown window and fired another background prepend, progressively loading the
entire history of large sessions on open.

Commit an explicit empty [] snapshot for assistant messages so fetched-empty
counts as renderable, while non-assistant messages keep the absent
representation and its no-op commit behavior.

Reproduced and verified headless against a real 857-message session: before,
20 message fetches escalating to limit=857; after, one initial page and a
single progressive-mount prepend.

Author: btriapitsyn