refactor(web): compose app from route modules

[xingkaixin]

Summary

• move route-specific content and derived state into AppRouteContent
• extract route header titles and breadcrumbs into a pure model builder
• replace the wide sidebar prop surface with explicit model and action interfaces
• reduce App.tsx from 915 to 596 lines while reusing canonical session and agent indexes

Impact

This is an internal composition refactor. Existing routes, search, bookmarks, live refresh, keyboard navigation, and sidebar behavior remain unchanged.

Validation

• pnpm format
• pnpm lint
• pnpm format:check
• pnpm build
• pnpm test (662 tests)
• pnpm test:e2e (6 tests)

Closes CS-45