feat(i18n): support per-locale UI string overrides

[mathis6787]

Summary

Adds per-locale UI string overrides and localizes previously hardcoded Stardust interface text.

Before this change, i18n.strings was shared by every locale. Multi-locale sites could not provide different UI strings for each language without client-side JavaScript workarounds.

Changes

• Add an optional strings map to each locale.
• Merge partial locale strings with the shared i18n.strings fallback.
• Resolve strings using the active locale or configured default locale.
• Add localization keys for:
  • Search placeholder
  • Table-of-contents title
  • Code-copy labels and feedback
  • Copy-page-as-Markdown label
  • Dartdoc API and back-to-docs labels
• Use the existing search.searching string during search.
• Preserve the original image alt text in the zoom overlay.
• Update the JSON schema, generated Dart schema, and i18n documentation.
• Add configuration, rendering, Dartdoc, and CLI test coverage.

Example

i18n:
  enabled: true
  defaultLocale: en

  # Optional strings shared by all locales.
  strings:
    footer.poweredBy: "Documentation by"

  locales:
    - code: en
      label: English
      path: /

    - code: fr
      label: Français
      path: /fr/
      strings:
        nav.previous: "← Précédent"
        nav.next: "Suivant →"
        search.placeholder: "Rechercher…"
        search.searching: "Recherche…"
        toc.title: "Sur cette page"
        code.copy: "Copier"
        code.copyLabel: "Copier le code"
        code.copied: "Copié !"
        code.copyFailed: "Échec de la copie"
        page.copyMarkdown: "Copier en Markdown"

Potential follow-up improvements

This PR intentionally preserves SearchConfig.placeholder and
TocConfig.title to maintain compatibility with existing configurations.

In the future, it may be cleaner to move all user-facing defaults—including
"Search docs..." and "On this page"—into I18nStrings. SearchConfig
would then contain only search behavior, while TocConfig would contain only
table-of-contents behavior.

After centralizing all user-facing defaults in I18nStrings, a subsequent
improvement could move the built-in translations out of i18n_config.dart and
into external locale catalogs, for example:

stardust-theme-translations/
└── locales/
    ├── en.json
    ├── fr.json
    ├── es.json
    └── ar.json

I would be interested in working on these follow-up improvements if this
direction is accepted. I am also open to suggestions !