refactor(app): migrate session timeline to TanStack Virtual

[Hona]

Summary

• replace the patched virtua session timeline with TanStack Virtual chat-mode anchoring
• delegate prepend stability, append follow, end detection, and latest scrolling to TanStack APIs
• remove virtua-era scroll/cache compensation and the virtua patch/dependency
• keep Solid-rendered virtual rows keyed and measured safely across history prepends and row replacement

Validation

• bun typecheck in packages/app
• bun run build in packages/app
• bun typecheck in packages/desktop
• bun run build in packages/desktop
• git diff --check
• repository pre-push hook: bun turbo typecheck
• desktop smoke check in feat: Desktop - New Session: open at latest, scroll to history top without crash/overlap, and return via Latest

Notes

• adds @tanstack/solid-virtual and removes the patched virtua integration
• TanStack packages are excluded from minimumReleaseAge because the required chat/end-anchor APIs are in the newly resolved release