feat(learning): progressive step hints with gated solution reveal

[OthmaneZ05]

Summary of Changes

The roadmap player now surfaces the hints[] and solution fields that the roadmap format has carried since v1. Between the instruction and the failure message, a learner who is stuck gets a middle rung: progressive hints revealed one at a time, with the full solution as a distinct, gated last step.

• Progressive reveal, strictly sequential — one button reveals the next hint ("Show hint (2/3)"); there is no way to jump ahead. Revealed hints stay readable for the whole session, including when navigating between steps and back.
• Solution behind a light brake — after the last hint, the button switches to "Reveal the solution"; the first click arms a confirmation label, the second reveals. The armed state resets when the step changes. The solution card is styled distinctly (amber, key icon) from hint cards (accent, lightbulb).
• Hints are available without a prior validation attempt — a learner stuck before even trying needs the first nudge most.
• Steps with no hints and no solution render no hint controls at all.
• Reveal state lives in useLearningPlayer as revealedHintsByStepId (mirrors resultsByStepId: reset on roadmap open/close, kept across step navigation) — the natural seam for the upcoming local-progression persistence to pick up.
• The lightweight markdown renderer (renderInstruction + CodeBlock) moved from RoadmapPlayer.tsx to a shared InstructionMarkdown.tsx so hints and solutions render code fences exactly like instructions (mechanical extraction, no behavior change).
• i18n en/fr for all new labels. No backend changes.

Types of Changes

• New feature / node type addition
• Bug fix (non-breaking change resolving an issue)
• Refactoring / structural cleanup
• Documentation update

Verification & Testing

Automated Checks

• Run npm run lint successfully with no errors
• Run npm run build successfully with no compilation errors
• Run npm test successfully (all tests pass)

Manual Verification

Frontend suite: 213 tests green (10 new — StepHints.test.tsx covers ordered reveal, no skipping, the two-click solution brake, solution-only and hint-less steps, revisit with persisted state; useLearningPlayer.test.ts covers increment/clamp, per-step persistence across navigation, reset on open/close). Backend suite untouched and green (217).

End-to-end against real Docker (headless Chrome driving the app on the example roadmap): 17/17 checks — revealed hints survive step navigation, the solution brake arms then reveals, the arming resets when leaving the step, a 1-hint step without solution shows no solution button, and the FR locale renders all new labels.

Checklist

• My code follows the repository's code style and lint standards
• I have updated the documentation or instructions if necessary
• All unit and integration tests are passing