perf: eliminate next-themes and framer-motion from initial JS bundle

- Replace next-themes (38 KiB) with a tiny custom ThemeProvider (~< 1 KiB)
  using localStorage + classList.toggle for theme management
- Add FOUC-prevention inline script in layout.tsx to apply saved theme
  before React hydrates
- Remove framer-motion from Header.tsx: nav entry now uses CSS slideDown
  keyframe, mobile menu uses CSS opacity/translate transitions
- Remove framer-motion from ThemeToggle.tsx: use Tailwind hover/active scale
- Remove framer-motion from legal-notice and privacy-policy pages
- Update useTheme import in ThemeToggle to use custom ThemeProvider
- Add slideDown keyframe to tailwind.config.ts
- Update tests to mock custom ThemeProvider instead of next-themes

Result: framer-motion moves from "First Load JS shared by all" to lazy
chunks; next-themes chunk eliminated entirely; -38 KiB from initial bundle

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/__tests__/components/Header.test.tsx

@@ -25,10 +25,10 @@ describe('Header', () => {
     render(<Header />);
     expect(screen.getByText('dk')).toBeInTheDocument();
     
-    // Check for navigation links
-    expect(screen.getByText('Home')).toBeInTheDocument();
-    expect(screen.getByText('About')).toBeInTheDocument();
-    expect(screen.getByText('Projects')).toBeInTheDocument();
-    expect(screen.getByText('Contact')).toBeInTheDocument();
+    // Check for navigation links (appear in both desktop and mobile menus)
+    expect(screen.getAllByText('Home').length).toBeGreaterThan(0);
+    expect(screen.getAllByText('About').length).toBeGreaterThan(0);
+    expect(screen.getAllByText('Projects').length).toBeGreaterThan(0);
+    expect(screen.getAllByText('Contact').length).toBeGreaterThan(0);
   });
 });