Add sitewide header language selector with route-preserving locale switching

[Copilot]

This PR adds a sitewide language selector to the top navigation so users can switch locale from any page. Locale changes now keep users on the equivalent route instead of sending them to locale roots.

• Header navigation: language selector

  • Adds a locale dropdown in header.jsx powered by SUPPORTED_LOCALES / LOCALE_NAMES.
  • Uses current locale from LocaleContext.
  • On selection change, navigates to a locale-adjusted path rather than a fixed landing page.

• Locale routing: path rewrite helpers

  • Adds getLocaleSwitchPath(pathname, locale) in i18n/index.js for bidirectional locale switching across:
    • non-exercise routes (/about ⇄ /fr-CA/about)
    • exercise routes (/exercises/en-US/python/E1/ ⇄ /exercises/fr-CA/python/E1/)
    • exercise root behavior (/exercises/ ⇄ /exercises/<locale>/)
  • Refines getLocalePath to preserve exercise subpaths when localizing paths.

• Localization strings + UI styling

  • Adds nav.language label translations in strings.js for en-US, fr-CA, and pt-BR.
  • Adds header/mobile styles for selector layout and spacing in style.css and mobile.css.

• Coverage updates

  • Extends i18n/index.spec.js with route-switching expectations for exercise and non-exercise paths, including default-locale edge behavior.

export function getLocaleSwitchPath(pathname, locale) {
  // preserves route structure while rewriting locale segment
  // e.g. /exercises/en-US/python/E1/ -> /exercises/fr-CA/python/E1/
}

[Copilot]

Pull request overview

Adds a locale-aware i18n layer and exposes it via a sitewide header language selector that preserves the current route when switching languages.

Changes:

• Introduces site/src/i18n helpers (getLocaleFromPath, getLocalePath, getLocaleSwitchPath) plus a LocaleContext provider/hook to supply locale + translated UI strings.
• Updates header/nav and multiple components to use localized labels (exercise UI strings, footer message, summary card terms).
• Adds locale-specific content pages (fr-CA, pt-BR), plus styling updates for the new selector and related layout.

Reviewed changes

Copilot reviewed 31 out of 31 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
site/src/utils/language.js	Defaults getCurrentLanguage locales from i18n constants.
site/src/templates/page.jsx	Passes heroId from frontmatter into Hero.
site/src/templates/index.jsx	Passes heroId from frontmatter into Hero.
site/src/templates/exercise.jsx	Uses i18n strings for “Exercise” navigation labels.
site/src/styles/style.css	Adds header layout styles for nav + language selector.
site/src/styles/mobile.css	Adjusts header controls layout on mobile.
site/src/styles/global.css	Reformatting + global style adjustments (also touches link selectors).
site/src/i18n/strings.spec.js	Adds tests to validate per-locale string key consistency + fallback behavior.
site/src/i18n/strings.js	Adds translated UI strings for nav/footer/exercise/summary cards.
site/src/i18n/LocaleContext.jsx	Adds LocaleProvider/useLocale to expose locale + strings.
site/src/i18n/index.spec.js	Adds tests for locale detection/path rewriting helpers.
site/src/i18n/index.js	Implements locale constants, string lookup, and route rewrite helpers.
site/src/components/translations.jsx	Displays human-friendly locale names in translations list.
site/src/components/summary-card.jsx	Localizes SummaryCard term labels via LocaleContext.
site/src/components/site-map.jsx	Localizes “Level” headings via LocaleContext.
site/src/components/layout.jsx	Wraps app layout in LocaleProvider.
site/src/components/hero.jsx	Supports stable heroId-based hero class naming.
site/src/components/header.jsx	Adds locale dropdown and localized nav labels with route-preserving switching.
site/src/components/footer.jsx	Localizes footer text via LocaleContext.
site/src/components/exercises.jsx	Localizes exercise list labels via LocaleContext.
site/src/components/exercise-nav.jsx	Localizes “Level” headings via LocaleContext.
site/content/pt-BR/teach.mdx	Adds pt-BR localized Teach page content + heroId.
site/content/pt-BR/makerspace.mdx	Adds pt-BR localized Makerspace page content + heroId.
site/content/pt-BR/continue.mdx	Adds pt-BR localized Continue page content.
site/content/pt-BR/about.mdx	Adds pt-BR localized About page content + heroId.
site/content/fr-CA/teach.mdx	Adds fr-CA localized Teach page content + heroId.
site/content/fr-CA/makerspace.mdx	Adds fr-CA localized Makerspace page content + heroId.
site/content/fr-CA/continue.mdx	Adds fr-CA localized Continue page content.
site/content/fr-CA/about.mdx	Adds fr-CA localized About page content + heroId.
site/content/exercises/pt-BR/index.mdx	Adds heroId for pt-BR Learn index page.
site/content/exercises/fr-CA/index.mdx	Adds heroId for fr-CA Learn index page.

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