Add child-agent filters to the chat stream

[selfcontained]

Summary

• add a Chat filters popover beside the Chat/Console control
• add a default-on “Show child agents” filter that hides both parent→child and child→parent messages
• strengthen the selected Chat/Console segment styling
• add accurate filtered-empty recovery, accessible active-filter state, visible-only scroll tracking, and 44px coarse-pointer targets

Validation

• pnpm run check
• pnpm run finalize:web
• pnpm run test:e2e (195 passed, 13 expected skips)
• focused Vitest coverage (27 passed)
• Playwright validation on desktop and 390×844 mobile
• frontend UX review #1104: all four findings fixed and reviewer-verified