🎨 Palette: Add aria-current to active navigation links

[saint2706]

💡 What: Added aria-current="page" to active navigation links in the Sidebar and Navbar.
🎯 Why: Screen reader users rely on this attribute to know which link in a navigation menu corresponds to the current page. Visual indicators (like bold text or background color) are not sufficient for non-visual users.
♿ Accessibility:

• Sidebar links (Home, Curriculum, Progress, Exercises, Review) now have aria-current="page" when active.
• Sidebar accordion links (Phase Overview, Lessons) now have aria-current="page" when active.
• Navbar links (Home, Curriculum, Search) now have aria-current="page" when active.
  ✅ Verification:
• Added unit tests in src/components/__tests__/Navigation.test.tsx which verify the presence of the attribute.
• Verified with Playwright script (screenshot confirmed active state visually, script confirmed attribute presence).

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PR created automatically by Jules for task 3684024909551988662 started by @saint2706

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