fix(app): hide unsettled cold timeline

[Brendonovich]

Summary

• hide cold pinned timeline rows while estimated heights settle
• wait for mounted Markdown to apply converted HTML before revealing the timeline
• confirm the settled range and re-anchor at the bottom before first paint
• fade in only timelines that were withheld during cold loading
• isolate follow and jump state per session before each virtualizer remount
• avoid flashing partial or older rows and post-mount scroll corrections during session switches

Stack

• depends on fix(app): stabilize initial timeline rendering #44333

Validation

• bun typecheck (packages/app)
• bunx playwright test e2e/regression/session-timeline-history-root.spec.ts e2e/regression/session-timeline-transport.spec.ts: 10 passed
• focused history-root rerun: 2 passed
• push hook: full monorepo typecheck