Migrate to HeroUI v3 and Tailwind CSS v4

- Upgrade from Tailwind CSS v3 to v4 with CSS-first configuration
- Install HeroUI v3 beta packages (@heroui/react, @heroui/styles)
- Migrate PostCSS config to new @tailwindcss/postcss plugin
- Convert tailwind.config.cjs to CSS @theme directive in globals.css
- Replace @tailwindcss/typography with custom prose styles

Component migrations:
- theme-toggle, back-to-top: HeroUI Button with onPress
- post-card, post-list-item: HeroUI Card compound components
- right-sidebar: HeroUI Card, Avatar, Chip
- search-modal: HeroUI Modal with compound structure
- nav-menu: HeroUI Button for mobile controls
- post-list-with-controls: HeroUI Button for sorting/pagination

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

components/theme-toggle.tsx

@@ -3,6 +3,7 @@
 import { useEffect, useState } from 'react';
 import { useTheme } from 'next-themes';
 import { FiMoon, FiSun } from 'react-icons/fi';
+import { Button } from '@heroui/react';
 
 export function ThemeToggle() {
   const { theme, setTheme } = useTheme();
@@ -20,17 +21,18 @@ export function ThemeToggle() {
   const isDark = theme === 'dark';
 
   return (
-    <button
-      type="button"
-      className="inline-flex h-9 w-9 items-center justify-center rounded-full text-accent-textLight transition duration-180 ease-snappy hover:-translate-y-0.5 hover:bg-accent-soft hover:text-accent active:scale-95 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-accent/40 dark:text-accent-textDark dark:hover:bg-slate-800 dark:hover:text-accent"
-      onClick={() => setTheme(next)}
+    <Button
+      isIconOnly
+      variant="ghost"
+      className="h-9 w-9 rounded-full text-[var(--color-accent-text-light)] transition duration-180 hover:-translate-y-0.5 hover:bg-[var(--color-accent-soft)] hover:text-[var(--color-accent)] active:scale-95 dark:text-[var(--color-accent-text-dark)] dark:hover:bg-slate-800 dark:hover:text-[var(--color-accent)]"
+      onPress={() => setTheme(next)}
       aria-label={theme === 'dark' ? '切換為淺色主題' : '切換為深色主題'}
     >
       {isDark ? (
-        <FiSun className="h-4 w-4 rotate-0 text-amber-400 transition-transform duration-260 ease-snappy" />
+        <FiSun className="h-4 w-4 rotate-0 text-amber-400 transition-transform duration-260" />
       ) : (
-        <FiMoon className="h-4 w-4 rotate-180 text-blue-500 transition-transform duration-260 ease-snappy" />
+        <FiMoon className="h-4 w-4 rotate-180 text-blue-500 transition-transform duration-260" />
       )}
-    </button>
+    </Button>
   );
 }