feat(01-04): add TanStack Virtual list virtualization and keyboard navigation hook

- Install @tanstack/react-virtual for virtualized list rendering
- Create useRovingTabIndex hook in src/renderer/lib/keyboard.ts for arrow-key navigation
- Add virtualization to 6 lists: TaskQueue, ApprovalList, ActivityList, CostList, AgentRoster, MessageList
- MessageList uses measureElement for variable-height message items with auto-scroll
- All 6 lists have role="listbox" with role="option" on rows for accessibility
- All 6 lists support ArrowDown/ArrowUp/j/k/Home/End keyboard navigation
- Add 8 unit tests for useRovingTabIndex covering all key bindings and edge cases

package.json

@@ -32,6 +32,7 @@
     "@agentcompany/adapter-utils": "workspace:*",
     "@fontsource-variable/jetbrains-mono": "5.2.8",
     "@phosphor-icons/react": "2.1.10",
+    "@tanstack/react-virtual": "^3.13.23",
     "clsx": "2.1.1",
     "electron-log": "5.4.3",
     "electron-updater": "6.8.3",