Add Accept and User-Agent headers, improve HTML error detection

**Motivations:**
- Some NIP-95 endpoints require Accept and User-Agent headers
- Better diagnosis of HTML error responses (404, 403, 500)
- More detailed error messages for troubleshooting

**Root causes:**
- Missing Accept: application/json header may cause endpoints to return HTML
- Missing User-Agent header may cause some servers to reject requests
- HTML error pages need better detection and classification

**Correctifs:**
- Added Accept: application/json header to all requests
- Added User-Agent: zapwall.fr/1.0 header to all requests
- Improved HTML error detection with 404/403/500 classification
- Enhanced error messages with specific suggestions based on error type
- Increased body preview in logs from 200 to 500 characters for better diagnosis

**Evolutions:**
- None

**Pages affectées:**
- pages/api/nip95-upload.ts

pages/api/nip95-upload.ts

@@ -82,6 +82,10 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
           const clientModule = isHttps ? https : http
           const headers = requestFormData.getHeaders()
 
+          // Add standard headers that some endpoints require
+          headers['Accept'] = 'application/json'
+          headers['User-Agent'] = 'zapwall.fr/1.0'
+
           // Add NIP-98 Authorization header if token is provided
           if (authToken) {
             headers['Authorization'] = `Nostr ${authToken}`
@@ -253,16 +257,43 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
       const h1Match = response.body.match(/<h1[^>]*>([^<]+)<\/h1>/i)
       const errorText = titleMatch?.[1] || h1Match?.[1] || 'HTML error page returned'
 
+      // Check if it's a 404 or other error page
+      const is404 = response.body.includes('404') || response.body.includes('Not Found') || titleMatch?.[1]?.includes('404')
+      const is403 = response.body.includes('403') || response.body.includes('Forbidden') || titleMatch?.[1]?.includes('403')
+      const is500 = response.body.includes('500') || response.body.includes('Internal Server Error') || titleMatch?.[1]?.includes('500')
+      
       console.error('NIP-95 proxy HTML response error:', {
         targetEndpoint,
         finalUrl: currentUrl.toString(),
         status: response.statusCode,
         errorText,
-        bodyPreview: response.body.substring(0, 200),
+        is404,
+        is403,
+        is500,
+        bodyPreview: response.body.substring(0, 500),
+        contentType: 'HTML (expected JSON)',
+        suggestion: is404 
+          ? 'The endpoint URL may be incorrect or the endpoint does not exist'
+          : is403
+          ? 'The endpoint may require authentication or have access restrictions'
+          : is500
+          ? 'The endpoint server encountered an error'
+          : 'The endpoint may not be a valid NIP-95 upload endpoint or may require specific headers',
       })
       
+      let userMessage = `Endpoint returned an HTML error page instead of JSON`
+      if (is404) {
+        userMessage = `Endpoint not found (404). The URL may be incorrect: ${currentUrl.toString()}`
+      } else if (is403) {
+        userMessage = `Access forbidden (403). The endpoint may require authentication or have restrictions.`
+      } else if (is500) {
+        userMessage = `Server error (500). The endpoint server encountered an error.`
+      } else {
+        userMessage = `Endpoint returned an HTML error page instead of JSON. The endpoint may be unavailable, the URL may be incorrect, or specific headers may be required. Error: ${errorText}`
+      }
+      
       return res.status(500).json({
-        error: `Endpoint returned an HTML error page instead of JSON. The endpoint may be unavailable or the URL may be incorrect. Error: ${errorText}`,
+        error: userMessage,
       })
     }