feat(theme-selector): underline and strikethrough on package faces

Package faces gain underline and strike toggles alongside bold and italic. Each face object carries the two new booleans, the face table shows U and S buttons (the S button itself renders struck through, U underlined), the preview applies text-decoration so links underline and strike faces strike for real, and theme.json round-trips both fields. Link and strike-through faces across shr, mu4e, erc, slack, and telega are seeded with the right decoration so they look correct by default. The schema self-test still passes, so old exports without the fields import unchanged.

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