feat(lessons): structured learning system with validation engine, lesson dock and Home routing

[srfwb]

Summary

Milestone 3: adds a complete lesson and challenge system to WeCode. Users can follow guided lessons (files locked, checkpoints validated in real time) and attempt free-form challenges (files editable, pass/fail checkpoints). The existing IDE shell is reused via a React context provider — project mode is untouched.

Ships with 1 lesson ("La structure d'une page HTML", 5 checkpoints) and 1 challenge ("Crée une page simple", 3 checkpoints) as proof of concept.

What changed

User-facing

• Home rail is now a router: Accueil / Leçons / Challenges tabs with a live progress counter on Leçons.
• Clicking a lesson card opens the IDE in lesson mode: starter files pre-loaded, file tree locked (no create/delete/rename), lesson dock visible with instructions and checkpoints.
• Checkpoints validate in real time as the user edits (300 ms debounce). Passed checkpoints turn green with a "fait" label.
• Challenges allow file operations and have pass/fail checkpoints.
• "Reprends là où tu t'étais arrêté" card on Home shows in-progress lessons (priority over projects).
• Clicking the WeCode brand in lesson mode returns to Home.
• Status bar shows checkpoint progress instead of version number in lesson mode.
• Changelog modal: "Nouveautés de la vX.Y.Z" link in the rail footer opens an in-app changelog viewer sourced from src/changelog.md.
• Template selector focus ring suppressed (no more parasitic amber outline).

Internal

• Validation engine (src/lessons/validation/): 16 rule types as a discriminated union, 6 implemented (element-exists, file-contains, file-not-contains, css-property, nesting, composite), pure functions testable under Node. Unimplemented types warn at runtime.
• Lesson data format: JSON files in src/lessons/data/ with starter files, steps, paragraphs and checkpoint definitions.
• useLessonStore (Zustand): manages active lesson, checkpoint states, start/complete/reset/exit lifecycle. Calls syncVfsNow on start so the preview iframe loads immediately.
• useHomeStore (Zustand): tab routing for the Home rail.
• LessonProvider (React context): wraps IdeShell in lesson mode, wires VFS change events to the validation engine with 300 ms debounce, persists completion via markLessonCompleted.
• LessonDock rewritten to read from context (mock deleted). CSS matches the Claude Design handoff. Dock-head is a semantic <button> with aria-expanded.
• IDE adaptations: FileTree respects fileOpsLocked, Toolbar exits lesson, StatusBar shows progress, shortcuts work in lesson view (Ctrl+N blocked).
• 17 commits, 154 vitest tests (25 files), 21 cargo tests. Two code reviews (superpowers + drafter) with all critical/important findings fixed.