Perkuat regression accessibility website

[man612]

Implement Phase 5 dari docs/UX-DIRECTION.md untuk issue #51.

Perubahan utama:

• tambahkan core contrast contract untuk text, muted text, accent, focus ring, dan inverse surface;
• kunci explicit :focus-visible ring dan prefers-reduced-motion contract;
• uji keyboard traversal/activation pada pack picker, course disclosure, dan Project Instructions action;
• uji focus-not-obscured terhadap sticky header/step rail;
• uji reduced-motion dengan browser context nyata;
• perluas responsive regression ke 320, 360, 390, 430, dan 820px;
• pertahankan target sentuh mobile >=44px serta no-horizontal-overflow pada primary flow.

Riset/acuan utama: WCAG 2.2 Focus Not Obscured, Reflow 320 CSS px, Target Size (Minimum), serta kontrak accessibility di docs/UX-DIRECTION.md.

Local validation:

• browser regression dua kali: OK;
• JSON Schema 7 schema / 18 instance: OK;
• repository/scope/README/site/CI contracts: OK;
• source freshness 28 source + 8 claim: OK;
• eval wiring Semester 1/2/3: OK;
• git diff --check: OK.

Related: #51.