refactor(ui): refine product information surfaces

[GCWing]

Summary

• make the shared IconButton quiet variant transparent at rest while keeping emphasized variants opaque
• refine the About dialog, Agent Harness overview, and Notification Center with shared UI primitives and clearer information hierarchy
• update localized copy and preserve the latest notification dialog accessibility/action contracts

Validation

• pnpm.cmd run design-system:check
• pnpm.cmd run check:web
• pnpm.cmd run i18n:audit
• pnpm.cmd --dir src/web-ui run test:run src/shared/notification-system/components/NotificationCenter.test.tsx src/app/scenes/agents/AgentsScene.test.tsx (2 files, 10 tests)
• git diff --check origin/1.0.0-explore...HEAD

Verification boundaries

• No browser automation, mock visual validation, native E2E, or manual rendered acceptance was run.
• No remote-workspace, remote-control, Peer Device Mode, or Detached Dispatch path was exercised; these are presentation-only changes.
• docs/features/startup-onboarding-design.md remains untracked locally and is not part of this PR.