Rewrite generate.py to match generate.java with full i18n support

Bring generate.py up to parity with generate.java (the source of truth).
Previously the Python script only produced English output and was missing
many features added to the Java generator.

Added:
- Full multi-locale build support (--locale, --all-locales flags)
- UI string loading from translations/strings/ with English fallback
- Content translation merging (resolveSnippet) from translations/content/
- Dotted-key token regex (e.g. {{nav.allPatterns}})
- Multi-pass token replacement (3 passes for nested tokens)
- Locale picker, hreflang links, and i18n script block rendering
- Contribute issue URLs (code, translation, suggest)
- Locale-aware URLs in nav arrows, related cards, and index cards
- i18n-aware difficultyDisplay and supportBadge lookups
- Locales loaded from html-generators/locales.properties

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges