[Feature] Backfill Storybook Story Coverage for CubeInput, CubeSkeleton, and CubePasswordInput

[lizzy-liang-bigstack]

Description

A brief summary to help everyone understand this feature.

Of the components already migrated into @bigstack-oss/cube-ui, there are some components haven't had storybook yet, including CubeInput, CubeSkeleton, and CubePasswordInput.

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.

• CubeInput has a story covering its documented variants/states (default, disabled, error/invalid, with leading/trailing icon, loading skeleton)
• CubeSkeleton has a story covering its shape/size variants
• CubePasswordInput has a story covering the show/hide-password toggle and its other input states
• All three follow the existing docs-style story pattern (StoryLayout/CaptionText/Heading helpers from internals/components), consistent with the CubeButton and CubeLoadingSpinner stories
• pnpm ui:dev and pnpm build:storybook run clean with no console errors/warnings on the new stories
• pnpm lint and pnpm test remain green

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).