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/components/Header.tsx

@@ -1,7 +1,6 @@
 "use client";
 
 import { useState } from "react";
-import { motion, AnimatePresence } from "framer-motion";
 import { Menu, X } from "lucide-react";
 import Link from "next/link";
 import { useLocale, useTranslations } from "next-intl";
@@ -26,11 +25,7 @@ const Header = () => {
   return (
     <>
       <div className="fixed top-8 left-0 right-0 z-50 flex justify-center px-6 pointer-events-none">
-        <motion.nav
-          initial={{ y: -100, opacity: 0 }}
-          animate={{ y: 0, opacity: 1 }}
-          className="pointer-events-auto bg-white/70 dark:bg-stone-900/70 backdrop-blur-2xl border border-white/40 dark:border-white/5 shadow-[0_8px_32px_rgba(0,0,0,0.05)] rounded-full px-3 py-2 flex items-center gap-1 md:gap-4"
-        >
+        <nav className="animate-slide-down pointer-events-auto bg-white/70 dark:bg-stone-900/70 backdrop-blur-2xl border border-white/40 dark:border-white/5 shadow-[0_8px_32px_rgba(0,0,0,0.05)] rounded-full px-3 py-2 flex items-center gap-1 md:gap-4">
           {/* Logo Pill */}
           <Link
             href={`/${locale}`}
@@ -63,7 +58,7 @@ const Header = () => {
               {locale === "en" ? "DE" : "EN"}
             </Link>
             <ThemeToggle />
-            
+
             {/* Mobile Menu Toggle */}
             <button
               onClick={() => setIsOpen(!isOpen)}
@@ -72,33 +67,30 @@ const Header = () => {
               {isOpen ? <X size={14} /> : <Menu size={14} />}
             </button>
           </div>
-        </motion.nav>
+        </nav>
       </div>
 
       {/* Mobile Menu Overlay */}
-      <AnimatePresence>
-        {isOpen && (
-          <motion.div
-            initial={{ opacity: 0, y: 10, scale: 0.98 }}
-            animate={{ opacity: 1, y: 0, scale: 1 }}
-            exit={{ opacity: 0, y: 10, scale: 0.98 }}
-            className="fixed top-24 left-6 right-6 z-40 bg-white/90 dark:bg-stone-900/95 backdrop-blur-3xl border border-white/40 dark:border-white/10 rounded-[2.5rem] shadow-2xl p-6 md:hidden overflow-hidden"
-          >
-            <div className="flex flex-col gap-3">
-              {navItems.map((item) => (
-                <Link
-                  key={item.name}
-                  href={item.href}
-                  onClick={() => setIsOpen(false)}
-                  className="px-6 py-4 text-sm font-black uppercase tracking-[0.2em] text-stone-900 dark:text-stone-100 bg-stone-50 dark:bg-white/5 rounded-2xl transition-colors hover:bg-liquid-mint/10"
-                >
-                  {item.name}
-                </Link>
-              ))}
-            </div>
-          </motion.div>
-        )}
-      </AnimatePresence>
+      <div
+        className={`fixed top-24 left-6 right-6 z-40 bg-white/90 dark:bg-stone-900/95 backdrop-blur-3xl border border-white/40 dark:border-white/10 rounded-[2.5rem] shadow-2xl p-6 md:hidden overflow-hidden transition-all duration-200 ${
+          isOpen
+            ? "opacity-100 translate-y-0 pointer-events-auto"
+            : "opacity-0 -translate-y-2 pointer-events-none"
+        }`}
+      >
+        <div className="flex flex-col gap-3">
+          {navItems.map((item) => (
+            <Link
+              key={item.name}
+              href={item.href}
+              onClick={() => setIsOpen(false)}
+              className="px-6 py-4 text-sm font-black uppercase tracking-[0.2em] text-stone-900 dark:text-stone-100 bg-stone-50 dark:bg-white/5 rounded-2xl transition-colors hover:bg-liquid-mint/10"
+            >
+              {item.name}
+            </Link>
+          ))}
+        </div>
+      </div>
     </>
   );
 };