[Feature] Migrate Wave 9: Complex Composites (Header, SideBar, SearchBar, DatePicker, Modal, Upload, BackButton)

[lizzy-liang-bigstack]

Description

A brief summary to help everyone understand this feature.

The final wave, pulling together components from every prior wave. SideBar, SearchBar, and DatePicker are the largest remaining components in source.

Migrate each to its Cube* equivalent in @bigstack-oss/cube-ui following the existing pattern: reame, split styles/utils as needed, colocated test, Storybook story, changeset.

Definition of Done (DoD)

A list of technical requirements must be met for completion. These may include, but are not limited to:

• API data conforming to a specified structure.
• An UI delivering defined functionalities.
• Passing all unit tests.
• Passing a code review.

• CubeHeader migrated
• CubeSideBar migrated
• CubeSearchBar migrated
• CubeDatePicker migrated
• CubeModal migrated
• CubeUpload migrated
• CubeBackButton migrated
• Each component has a colocated .test.tsx
• Each component has a Storybook story
• Each component is re-exported from src/index.ts
• pnpm lint, pnpm test, and pnpm build are green
• Changeset(s) added covering all 6 components for the next release

Technical Notes

A section for tech leads to to document technical risks and/or any concerns regarding the DoD.

• Feature Owner:
• Frontend:
• Backend:
• Infra:

UI/UX Mockups (Optional)

Links to design prototypes or wireframes (if applicable).

None

Output artifacts (Definition of Done)

Beyond code, docs, and config changes, completing this issue must also deliver:

• Handbook knowledge update — land the durable, team-readable knowledge from this
  work into the bigstack-handbook cubecos kb (kb/cubecos/…) via
  /bigstack-core:save-to-handbook (Topic / Runbook / Known-issue / ADR as fits).