i18n: add Italian (it) interface

[alb308]

Summary

Adds Italian as an interface language. Every label of the reference table (381 of 381) is translated in crates/ui-egui/src/i18n/it.rs. it, it-IT, it-CH and it_IT.UTF-8 system locales pick Italian, and View › Interface language and app.language.set {"language":"it"} switch to it.

• Ribbon tabs, groups and commands use the names Italian Excel users know (Home, Inserisci, Layout di pagina, Formule, Dati, Revisione, Visualizza; Incolla, Unisci e allinea al centro, Formattazione condizionale, Blocca riquadri, Somma automatica…). These are feature names, which AGENTS.md allows. Descriptions and messages are written from the meaning of the English.
• Formulas and number formats stay en-US in this PR. Italian formulas come in a follow-up stacked on M12: formulas in German, Brazilian Portuguese, Russian and French, full Spanish table #443 (see below).
• Wiring follows the Polish PR i18n: add Polish (pl) interface #650: Language::It, its code and name ("Italiano"), the locale tags, the app.language.set help text, the snapshot example's --language list and the theme/interaction tests.
• Credits: adds my own entry to contributors/people.toml.

Screenshots (offscreen snapshot example, --language it): the Home and Formule tabs render in Italian, with no truncated labels. Watch\nWindow and AutoSave stay English, as in every language, because they don't go through tr() yet.

Tests

• i18n::tests gains Italian checks (every reference key present, Data → Dati, the Page Layout|Orientation split, it-IT / it_CH.UTF-8 tags).
• tests/formula_locale.rs: italian_interface_retains_its_canonical_formula_fallback.
• tests/interaction.rs: Italian in the ribbon language switch.

Ran locally (Rust 1.99):

• cargo fmt --all --check, cargo xtask assets, cargo xtask layers and cargo xtask wasm pass. cargo xtask ci gets past clippy.
• cargo test --workspace: everything passes except three failures that are already on main:
  • missing French: Different First Page (fix(i18n): French labels name unrelated actions and omit two required translations #529, fixed in Fix main's red CI: CSV General numbers save at full precision; fr i18n keys #692);
  • csv_save_keeps_the_separator_it_was_opened_with;
  • saving_an_opened_semicolon_csv_keeps_semicolons.
• With the French assertion skipped locally, the Italian i18n tests pass.

Review

The table was written with Claude (AI agent) on behalf of a native Italian speaker, who has not yet reviewed it line by line. Corrections from Italian Excel users are welcome.

🤖 Generated with Claude Code