[Story]: Highlight passed knowledge checks visibly

[adoLime]

User Story

As an onboardee, I want a clear, visible highlight the moment I pass a knowledge check, so that I have immediate, satisfying feedback that I understood the material and can move on.

Context & Motivation

• Type: Functional (Onboarding UX)
• Parent epic: 07 EPIC - Learning, Upskilling & Content Recommendation #56 (07 EPIC – Learning, Upskilling & Content Recommendation)
• Teams: team:frontend

Status quo: The backend already returns checkSummary.passed per phase. The frontend shows a small "Passed" chip and a green ClipboardCheck icon on the phase card. The PhaseCheckModal shows per-attempt result, and OnboardingCompleteCelebration triggers confetti only for the final phase's check.

Gap noticed: After passing a non-final knowledge check, the visual feedback is minimal: a small green chip and an icon, no
immediate celebration or persistent "phase completed" highlight.

Acceptance Criteria

• On a passed submit, the PhaseCheckModal shows a clear success state (e.g. icon + message).
• After closing the modal, the phase card visibly reflects the passed state, green accent border, success icon, "Phase completed" badge.
• In the onboarding path overview, passed phases get a distinct success marker (e.g. ✓ icon + "Completed" label).
• Passing a particularly relevant knowledge check (e.g. final phase of a milestone group. left to the implementing team to define which phases qualify) triggers a brief, non-blocking celebration (toast or mini-confetti, variant left to the implementing team); respects prefers-reduced-motion.
• State is conveyed by icon + text, not color alone; light/dark themes; keyboard focus stays visible; i18n (en/de); no false
  "Passed" indication on submit failure.