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>

package.json

@@ -70,7 +70,6 @@
     "lucide-react": "^0.542.0",
     "next": "^15.5.7",
     "next-intl": "^4.7.0",
-    "next-themes": "^0.4.6",
     "node-cache": "^5.1.2",
     "node-fetch": "^2.7.0",
     "nodemailer": "^7.0.11",