Show detailed error message on upload failure

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

artdag.py

@@ -534,7 +534,13 @@ def upload(filepath):
             if resp.status_code == 401:
                 click.echo("Authentication failed. Please login again.", err=True)
                 sys.exit(1)
-            resp.raise_for_status()
+            if resp.status_code >= 400:
+                try:
+                    detail = resp.json().get("detail", resp.text)
+                except:
+                    detail = resp.text
+                click.echo(f"Upload failed: {resp.status_code} - {detail}", err=True)
+                sys.exit(1)
             result = resp.json()
         click.echo(f"CID: {result['cid']}")
         click.echo(f"Size: {result['size']} bytes")