refactor(i18n): translate entire app from French to English

[srfwb]

Summary

Switches every user-facing string in WeCode from French to English. No i18n library — strings stay hardcoded, just in the new language. Internal type values (difficulty, HomeTab, palette group labels) also move to English identifiers.

What changed

User-facing

• All UI labels, buttons, placeholders, tooltips, aria-labels, toast messages, error messages, and modal text are now in English.
• Editor hover hints (~50 entries for HTML elements, attributes, and CSS properties) translated.
• Lesson and challenge content (titles, descriptions, instructions, checkpoint labels) translated.
• In-app changelog rewritten in English (Added / Changed / Fixed).
• Home greeting says "Welcome to WeCode" instead of "Bienvenue dans WeCode".
• Date format switched from fr-FR to en-US.

Internal

• LessonData.difficulty: "débutant" | "intermédiaire" | "avancé" → "beginner" | "intermediate" | "advanced".
• HomeTab: "accueil" → "home" (+ all consumers in HomeShell, HomeNav, palette shortcut).
• PALETTE_GROUP_LABELS: French labels → English.
• fileErrorMessages.ts: all 8 error mappings translated.
• statusBarFormat.ts: save status strings translated.
• Test assertions updated across 5 test files to match English strings.
• CLAUDE.md updated: "UI strings are in English".
• 12 commits, one per domain (types, home, IDE, projects, palette, hints, lessons, changelog, tests, docs).