feat(palette): wire real lessons into search and fix remaining French strings

[srfwb]

Summary

Two fixes: the command palette now shows real lessons and challenges instead of a placeholder, and 8 French strings that were missed during the translation pass are now in English.

What changed

User-facing

• The command palette's Lessons group shows the actual lessons and challenges (title, difficulty, estimated time) instead of a generic "Lesson path / Coming soon" placeholder. Clicking a result starts the lesson directly from the search.
• Template section heading translated: "Commence un nouveau projet" → "Start a new project".
• Template titles translated: "Dossier vierge" → "Blank project", "Démarrage HTML + CSS" → "HTML + CSS Starter".
• Bottom tip translated: "Astuce. Survole n'importe quel mot…" → "Tip. Hover any keyword…".

Internal

• src/palette/sources.ts — buildLessonItems() now maps over ALL_CONTENT from the lesson registry instead of returning a hardcoded placeholder. Each item calls useLessonStore.getState().startLesson(id) on select. Removed unused toast import.
• src/projects/templates.ts — 4 French strings translated.
• src/home/sections/TemplatesSection.tsx — heading translated.
• src/home/sections/BottomTip.tsx — 3 French strings translated.