feat(learning): pedagogical validation feedback with pass/fail/error states

[OthmaneZ05]

Summary of Changes

The roadmap player previously rendered validation results raw ([pass]/[fail] text, validator type slugs, raw error codes). This PR turns them into real pedagogical feedback:

• Per-validator cards with a ✓/✗/⚠ icon, colored left border, the backend message as primary text, and Expected/Observed values in monospace.
• Aggregate step banner with three distinct families: green success (with an explicit Next step button, or a "roadmap complete" message on the last step), red pedagogical failure (encouraging tone), and amber infrastructure error — visually and verbally impossible to confuse with a learner mistake, with a Docker-specific wording when the daemon was unreachable.
• Three-state step-list markers (✓/✗/⚠) via a shared stepOutcome helper: an infra error never shows as a learner failure. Error wins over fail in the aggregate because the step's real state is unknowable when a check couldn't run (documented in validationStatus.ts).
• Shared status presets (icon/color/label) live once in features/learning/validationStatus.ts; icons carry role="img" + aria-label so status is announced by screen readers.
• New --color-warning* CSS variables aligned with the existing amber infrastructure-error language (daemon banner, node badge, toast).
• New i18n keys (en/fr) for the banner copy, CTA, and marker labels.

No backend changes; no changes to useLearningPlayer (revalidation already replaces the whole per-step response, so no stale artifacts by construction).

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

Drove the full flow in a real browser (Playwright + Chrome) against real Docker on the example roadmap:

1. Pedagogical fail: validated step 1 on an empty project → red banner, ✗ card with message + Expected/Observed, red marker in the step list.
2. Success + revalidation: created and started the web container, revalidated → green banner + Next step button, no artifacts from the failed attempt; CTA advances to step 2.
3. Infrastructure error: with the Docker daemon unreachable, validated → amber banner with the Docker-specific "not your fault" wording, ⚠ cards with "Check couldn't run", amber marker; coexists with the global daemon banner. After restoring Docker, revalidated in the same session → clean recovery, no amber leftovers.
4. Spot-checked all new strings in French.

Frontend suite: 203 tests green (8 new: three result families, aggregation precedence, fully-validated step, revalidation without artifacts, Next step CTA).

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