[codex] Expose supported Modal panel styling

[JSv4]

Summary

• Add panelClassName as a public Modal panel styling hook.
• Make the existing panel-targeted className and style behavior explicit in the Modal props.
• Add coverage proving panel props land on the dialog panel while overlayClassName stays on the backdrop.
• Document the supported panel styling pattern and add a Storybook example for custom panel width.

Rationale

Issue #29 correctly identifies that consumers should not have to target the internal .oc-modal selector. The current implementation already forwarded className and style to the panel, so this keeps that existing contract instead of adding a redundant maxWidth prop. The new panelClassName provides a named escape hatch that mirrors overlayClassName and makes the target unambiguous.

Closes #29

Validation

• npx vitest run src/Modal/Modal.test.tsx
• npx vitest run src/Modal/Modal.test.tsx src/Dropdown/Dropdown.test.tsx src/Table/Table.test.tsx
• npx tsc --noEmit
• npm run build
• npm run lint (passes with existing warnings, no errors)