fix(app): correct turn visibility and simplify previews - #57682

Summary

Follow-up navigator fixes for beta after #57519 and #57522.

• Check actual rendered prompt bubbles against the header/composer-clipped viewport instead of accepting cached virtual-row estimates as proof of visibility. Keep highlighting all visible prompts; the reply-only fallback no longer selects a prompt solely because its invisible copy controls remain in view.
• Remove the tooltip's nested gray prompt background and both gradient text masks. Show up to four full prompt lines and five reply lines at a 20px line height, with ellipses only on overflow.
• Widen the card to 384px while bounding it to the chat pane, including narrow/RTL layouts; increase the preview text budget and update vertical bounds.
• Make decorative tick spans ignore pointer events so they cannot intercept adjacent tick targets in a dense rail.
• Consolidate header/composer clipping coverage in the new reading-viewport keeper instead of relying on the previous fixed-height fixture's assumption that the second prompt is offscreen.

This does not implement a dense-session overview or change the hover timing. Kit's separate #57672 addresses density and hover behavior; his branch is untouched. Our local branch was renamed to avoid its remote name collision.

Verification

• bun run check passes; app typecheck passes.
• Session unit tests: 110 passed.
• Production-build timeline-history keeper: 9 passed, fully parallel, 5 workers, no retries.
• General navigation and new viewport/preview keepers: 20 passed across 10 repeats at 5 workers, no retries.
• Verified the new preview keeper fails on unchanged beta's gradient mask.
• Reviewed before/after, narrow, and dark/RTL screenshots. The narrow RTL preview stays within the chat pane.
• Existing e2e typecheck errors remain in unchanged performance/regression files. The cold-entry benchmark times out before producing metrics on both base and this change; no performance comparison is claimed.
• Desktop preview verified against the existing service without changing repository dependencies or the backend.

Before

After