Unify new-thread composition and secondary-panel layout

[ymichael]

Summary

• add one shared new-thread composer that owns picker configuration, drafts, attachments, project switching, and request construction for both the root page and plugin SDK adapter
• add one shared secondary-panel layout that owns desktop sizing, compact drawer realization, native browser readiness, and hosted-pane registration for root compose and thread detail
• leave page-specific welcome, fork, machine setup, metadata, timeline, and macOS chrome behavior in thin wrappers
• centralize behavior-heavy tests and remove the resolved duplicated-assembly audit item

Testing

• pnpm exec turbo run typecheck --filter=@bb/app
• pnpm exec turbo run test --filter=@bb/app --force (341 files, 2,704 tests)
• pnpm exec turbo run build --filter=@bb/app

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