Add Spanish interface language, and make ribbon overflow visible

[GeronimoCastano]

Summary

• Adds Spanish (es) to File ▸ Options ▸ Interface language. Spanish system locales pick it up automatically.
• Neutral Spanish: tú for sentences, infinitives for commands, sentence case. Written from the English meaning. No Microsoft Word localisation was consulted or copied.
• When a ribbon tab is wider than the window, its edges show ‹ and › chevrons that scroll the row. The scrollbar was hidden before, so groups past the edge were clipped with no sign they existed. Spanish labels make Review and Home overflow at 1440 pt.

Review

Translations reviewed by a native Spanish speaker.

Testing

• cargo test --workspace passes.
• cargo xtask assets, layers, and wasm pass.
• rustfmt --check is clean on the changed files.
• Rendered the Spanish UI headlessly (Inicio, Insertar, Disposición, Revisar). Clicking › and ‹ scrolls the ribbon both ways.

Known gaps

• Style gallery names (Normal, Heading 1, Title…) still show in English. They come from the document's style definitions, not the catalog.
• The chevron tooltips show in English in other languages until translated.
• cargo xtask ci fails at clippy on main with Rust 1.95 (xtask/src/layers.rs, crates/formats, crates/layout). This PR doesn't change those files.

[echelon]

Merged — thank you for the contribution, @GeronimoCastano! 🎉 I merged main into your branch: Brazilian Portuguese landed just before, so the language list now has six entries; I added Spanish rows for four strings that landed on main meanwhile (Apply Styles, the dark page option and the link-scheme message), and translations of your “Scroll ribbon left/right” labels to the other catalogs. Your ‹ › edge cues were preferred over another ribbon-overflow approach because they scroll both ways.

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