feat(core): add i18n localization system, Italian translation, and LanguageSelector component

[FrancescoCastaldi]

Description

This PR introduces native Internationalization (i18n), Italian Translation (it-IT), and the <LanguageSelector> component to Evidence.

Features

1. i18n Architecture & Locale Context (core/src/i18n):
   • Reactive LocaleState context with $state for dynamic language switching.
   • Translation helper t(key, params) and localized date/number formatting using Intl.DateTimeFormat and Intl.NumberFormat.
2. Italian Localization (it-IT):
   • Complete translations for date range presets (e.g. "Oggi", "Ieri", "Ultimi 7 giorni", "Questo mese", "Tutto il periodo", etc.).
   • UI controls translation (search placeholders, "Totale", "Subtotale", pagination "Pagina X di Y", "Scarica CSV", etc.).
3. <LanguageSelector> (language_selector) Component:
   • Interactive language picker supporting pills and buttons style (variant="pills|buttons").
   • Allows users/viewers to switch language on the fly (e.g., 🇬🇧 English, 🇮🇹 Italiano).
   • Localized calendar and date pickers instantly re-render in the selected locale.
4. Project Settings:
   • Added locale field to projectSettingsSchema (defaulting to en-US).
5. Unit Tests:
   • Added test suite for locale-context and LanguageSelector with 100% pass.

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[FrancescoCastaldi]

Hi @archiewood @hughess! 👋 This PR adds a complete i18n localization system, full Italian translation (it-IT), and the component. Whenever you have time, we'd appreciate your review!

[hughess]

Hi @FrancescoCastaldi, thanks for this PR. We have a translations system in the product - is there a reason that doesn't work for you?
https://docs.evidence.dev/features/translations

[FrancescoCastaldi]

@hughess The existing translation system is great for translating markdown pages and routing, but this PR focuses on the built-in component UI strings (like table pagination 'Page X of Y', search placeholders, CSV download labels, date picker preset names like 'Last 7 days') and adds the <LanguageSelector> component for switching the UI locale dynamically on the fly.