PAP-2645: Virtualize long issue chat threads

Add @tanstack/react-virtual and gate IssueChatThread on a 150-row
threshold. Thread shorter than the threshold still uses the direct
render path; longer threads window the merged feed via
useWindowVirtualizer with dynamic measurement and overscan, so the
document scroll continues to represent the full conversation height
without introducing a nested scrollbar.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

ui/package.json

@@ -46,6 +46,7 @@
     "@radix-ui/react-slot": "^1.2.4",
     "@tailwindcss/typography": "^0.5.19",
     "@tanstack/react-query": "^5.90.21",
+    "@tanstack/react-virtual": "^3.13.0",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
     "cmdk": "^1.1.1",
@@ -62,17 +63,17 @@
     "tailwind-merge": "^3.4.1"
   },
   "devDependencies": {
-    "@tailwindcss/vite": "^4.0.7",
     "@storybook/addon-a11y": "10.3.5",
     "@storybook/addon-docs": "10.3.5",
     "@storybook/react-vite": "10.3.5",
+    "@tailwindcss/vite": "^4.0.7",
     "@types/node": "^25.2.3",
     "@types/react": "^19.0.8",
     "@types/react-dom": "^19.0.3",
     "@vitejs/plugin-react": "^4.3.4",
+    "storybook": "10.3.5",
     "tailwindcss": "^4.0.7",
     "typescript": "^5.7.3",
-    "storybook": "10.3.5",
     "vite": "^6.1.0",
     "vitest": "^3.0.5"
   }