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/ThemeToggle.tsx

@@ -2,8 +2,7 @@
 
 import * as React from "react";
 import { Moon, Sun } from "lucide-react";
-import { useTheme } from "next-themes";
-import { motion } from "framer-motion";
+import { useTheme } from "./ThemeProvider";
 
 export function ThemeToggle() {
   const { theme, setTheme } = useTheme();
@@ -18,11 +17,9 @@ export function ThemeToggle() {
   }
 
   return (
-    <motion.button
-      whileHover={{ scale: 1.05 }}
-      whileTap={{ scale: 0.95 }}
+    <button
       onClick={() => setTheme(theme === "dark" ? "light" : "dark")}
-      className="p-2 rounded-full bg-stone-100 dark:bg-stone-800 text-stone-800 dark:text-stone-100 hover:bg-stone-200 dark:hover:bg-stone-700 transition-colors border border-stone-200 dark:border-stone-700 shadow-sm"
+      className="p-2 rounded-full bg-stone-100 dark:bg-stone-800 text-stone-800 dark:text-stone-100 hover:bg-stone-200 dark:hover:bg-stone-700 transition-colors border border-stone-200 dark:border-stone-700 shadow-sm hover:scale-105 active:scale-95 transition-transform"
       aria-label="Toggle theme"
     >
       {theme === "dark" ? (
@@ -30,6 +27,6 @@ export function ThemeToggle() {
       ) : (
         <Moon size={18} className="text-stone-600" />
       )}
-    </motion.button>
+    </button>
   );
 }