WIP: Prototype right panel tab splitting

[ymichael]

Summary

• Prototypes Dockview-backed drag/drop splitting for right-panel tabs
• Keeps the compact drawer path on the existing tab strip
• Adds scoped CSS overrides so the dock header is closer to the previous panel chrome

WIP / known rough edges

• Visual parity is closer but still not final: header/panel spacing and Dockview group chrome need more UI QA
• Browser and terminal split panes still activate the live tab rather than duplicating live native surfaces
• Layout persistence is prototype-level and may need product decisions before shipping

Validation

• pnpm exec turbo run typecheck --filter=@bb/app
• pnpm exec turbo run test --filter=@bb/app

73 test files / 485 tests passed locally.