Complete remaining button and rail config knobs

[blackopsrepl]

Closes #10.

Summary

• make iconOnly buttons actually icon-only while keeping an accessible label
• implement badges support for rail resource cards instead of silently ignoring it
• document the completed rail badge and icon-only button behavior in README and WIREFRAME

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Guard badges against scalar strings before iterating

SF.rail.createCard() now assumes any truthy config.badges.length also supports forEach(). That breaks a single-string badge (badges: 'TEMPRA'): strings have a length, so this branch runs, but forEach is undefined and the card throws before rendering. Because type is already a scalar string and the new API docs describe badges as strings or styled objects, this is an easy input shape for callers to hit.

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