fix: align frontend with design spec + UX improvements

Design spec compliance:
- Fix body font-weight: font-medium → font-normal (spec requires 400)
- PostCard: add group hover effects, underline on title hover, arrow →, translate animation
- PostCard: add text-lg to excerpt (spec Body style)
- PostMeta: fix date color neutral-400 → neutral-500 (spec Ash color)
- Homepage: fix error color neutral-400 → red-600 (spec Error color)
- Register: replace green/yellow with black/gray (spec: no non-functional colors)
- Drafts: remove green from Publish button
- PostContent: unify pre background to #fafafa (spec Vapor color)
- ImageUpload: replace dashed border with solid thin line

Bug fixes:
- PostPage: use notFound() instead of router.push('/not-found')
- next.config.ts: add images.remotePatterns for external images

UX improvements:
- Editor: responsive grid-cols-1 md:grid-cols-2 for mobile
- Editor pages: replace alert() with SaveStatus component
- Autosave delay: 300ms → 2000ms to reduce API pressure

next.config.ts

@@ -2,6 +2,18 @@ import type { NextConfig } from "next";
 
 const nextConfig: NextConfig = {
   output: "standalone",
+  images: {
+    remotePatterns: [
+      {
+        protocol: "https",
+        hostname: "**",
+      },
+      {
+        protocol: "http",
+        hostname: "**",
+      },
+    ],
+  },
 };
 
 export default nextConfig;