To be able to fork a session from the CLI at a specified message

- Use direct Storage.write() calls instead of updateMessage/updatePart
- Match import/export pattern used elsewhere in codebase

Fixes #5088 by adding a new optional \`placeholderText\` theme color that allows
customizing the color of placeholder text in input fields and textareas.

Changes:
- Add \`placeholderText\` to ThemeColors type with fallback to \`textMuted\`
- Apply \`placeholderColor\` prop to <input> components in dialog-select
- Apply styled placeholder text to <textarea> components in dialog-prompt and prompt
- Update JSON schemas to include \`placeholderText\` property
- Maintain backward compatibility - existing themes work unchanged

The new field follows ariane-emory's suggestion and provides a minimal,
focused solution for theming placeholder text.

To be able to fork a session from the CLI at a specified message

- Use direct Storage.write() calls instead of updateMessage/updatePart
- Match import/export pattern used elsewhere in codebase