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>

tailwind.config.ts

@@ -16,9 +16,14 @@ export default {
           "0%": { opacity: "0", transform: "translateY(10px)" },
           "100%": { opacity: "1", transform: "translateY(0)" },
         },
+        slideDown: {
+          "0%": { opacity: "0", transform: "translateY(-20px)" },
+          "100%": { opacity: "1", transform: "translateY(0)" },
+        },
       },
       animation: {
         "fade-in": "fadeIn 0.5s ease-out",
+        "slide-down": "slideDown 0.4s ease-out",
       },
       colors: {
         background: "var(--background)",