Add 404 page and Web UI section to README

- Add styled 404 page for HTML requests (returns JSON for API calls)
- Add Web UI section to README documenting available routes

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

README.md

@@ -96,6 +96,20 @@ Keys are stored in `$ARTDAG_DATA/keys/`:
 
 **Important**: Private keys are gitignored. Back them up securely. Losing them invalidates all your signatures.
 
+## Web UI
+
+The server provides a web interface:
+
+| Path | Description |
+|------|-------------|
+| `/` | Home page with stats and README |
+| `/assets` | Browse registered assets |
+| `/activities` | View published activities |
+| `/users` | List registered users |
+| `/asset/{name}` | Asset detail page |
+| `/login` | Login page |
+| `/register` | Registration page |
+
 ## Client Commands
 
 ### Upload Media