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/ThemeProvider.tsx

@@ -1,11 +1,38 @@
 "use client";
 
-import * as React from "react";
-import { ThemeProvider as NextThemesProvider } from "next-themes";
+import React, { createContext, useContext, useEffect, useState } from "react";
 
-export function ThemeProvider({
-  children,
-  ...props
-}: React.ComponentProps<typeof NextThemesProvider>) {
-  return <NextThemesProvider {...props}>{children}</NextThemesProvider>;
+type Theme = "light" | "dark";
+
+const ThemeCtx = createContext<{ theme: Theme; setTheme: (t: Theme) => void }>({
+  theme: "light",
+  setTheme: () => {},
+});
+
+export function ThemeProvider({ children }: { children: React.ReactNode }) {
+  const [theme, setThemeState] = useState<Theme>("light");
+
+  useEffect(() => {
+    try {
+      const stored = localStorage.getItem("theme") as Theme | null;
+      if (stored === "dark" || stored === "light") {
+        setThemeState(stored);
+        document.documentElement.classList.toggle("dark", stored === "dark");
+      }
+    } catch {}
+  }, []);
+
+  const setTheme = (t: Theme) => {
+    setThemeState(t);
+    try {
+      localStorage.setItem("theme", t);
+    } catch {}
+    document.documentElement.classList.toggle("dark", t === "dark");
+  };
+
+  return <ThemeCtx.Provider value={{ theme, setTheme }}>{children}</ThemeCtx.Provider>;
+}
+
+export function useTheme() {
+  return useContext(ThemeCtx);
 }