test(app): add session timeline smoke coverage

[Hona]

Summary

• Add a generated session timeline smoke fixture covering text, reasoning, grouped context tools, edits, shell, web, question, and task parts.
• Add a headed smoke flow that scrolls through timeline history, verifies mounted/visible part ordering, and visits every turn through product hash navigation.
• Share the mock OpenCode server/error helpers and expose cursor headers so paged timeline coverage matches the app data path.

Verification

• Pre-push hook passed: bun turbo typecheck
• Previously passed during this perf pass: bun typecheck from packages/app
• Previously passed during this perf pass: bun typecheck from packages/ui
• Previously passed during this perf pass: bunx playwright test e2e/smoke/session-timeline.spec.ts --project=chromium --headed --trace on --reporter=line --workers=1

Notes

• Test-only selectors are added to production markup, but they do not change UI behavior.
• Visibility is based on DOM rect intersection with the timeline scroll viewport; grouped context rows count the represented child tool IDs when the group summary is visible.