feat(chat): migrate history list to @tanstack/react-virtual with deterministic mobile history loading

- Replace virtua with @tanstack/react-virtual for chat history on all
  surfaces: bottom anchoring (anchorTo: end), key-stable prepend
  preservation, and native iOS touch/momentum deferral live in the core
- Patch virtual-core to clamp the render range to real scroll bounds
  during transient adjustments
- Rows render in normal flow inside a translated wrapper so sticky user
  headers keep working; measurement snapshots cached per session
- Pre-write container height in scrollToFn so the browser cannot clamp
  anchor corrections to the stale height; hold the prepend anchor for up
  to 180 frames on mobile while fresh rows settle (cancelled by user
  input; desktop relies on core anchoring alone)
- Adaptive row-size estimate from per-session measured averages; disable
  reveal fade-in for virtualized history rows
- Mobile loads older history only through an explicit localized top
  button: no scroll-position trigger and no post-mount background
  prepend, so every insert happens from a resting state; a quiet-window
  hold defers any stray prepend commit while a touch gesture is active
- Desktop/VS Code keep the seamless scroll-up trigger and progressive
  background prepend

packages/ui/package.json

@@ -47,6 +47,7 @@
     "@pierre/diffs": "1.3.0-beta.6",
     "@replit/codemirror-vim": "^6.3.0",
     "@simplewebauthn/browser": "13.3.0",
+    "@tanstack/react-virtual": "3.14.5",
     "@xenova/transformers": "^2.17.2",
     "@zumer/snapdom": "^2.12.0",
     "beautiful-mermaid": "^1.1.3",