Improve HTML response detection in NIP-95 upload API

**Motivations:**
- Better error handling when endpoints return HTML error pages instead of JSON
- Provide clearer error messages to users

**Root causes:**
- Some NIP-95 endpoints (e.g., nostrimg.com) return HTML error pages instead of JSON
- Current error handling only catches JSON parse errors without detecting HTML responses
- Users get confusing error messages about JSON parsing when the real issue is an HTML response

**Correctifs:**
- Added HTML detection before JSON parsing
- Extract error message from HTML title or h1 tags when possible
- Provide clearer error message indicating HTML response instead of JSON
- Improved error logging with HTML body preview

**Evolutions:**
- None

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

pages/api/nip95-upload.ts

@@ -243,6 +243,29 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
       })
     }
 
+    // Check if response is HTML (error page) instead of JSON
+    const trimmedBody = response.body.trim()
+    const isHtml = trimmedBody.startsWith('<!DOCTYPE') || trimmedBody.startsWith('<html') || trimmedBody.startsWith('<!')
+    
+    if (isHtml) {
+      // Try to extract error message from HTML if possible
+      const titleMatch = response.body.match(/<title[^>]*>([^<]+)<\/title>/i)
+      const h1Match = response.body.match(/<h1[^>]*>([^<]+)<\/h1>/i)
+      const errorText = titleMatch?.[1] || h1Match?.[1] || 'HTML error page returned'
+      
+      console.error('NIP-95 proxy HTML response error:', {
+        targetEndpoint,
+        finalUrl: currentUrl.toString(),
+        status: response.statusCode,
+        errorText,
+        bodyPreview: response.body.substring(0, 200),
+      })
+      
+      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}`,
+      })
+    }
+
     let result: unknown
     try {
       result = JSON.parse(response.body)
@@ -254,7 +277,7 @@ export default async function handler(req: NextApiRequest, res: NextApiResponse)
         bodyPreview: response.body.substring(0, 100),
       })
       return res.status(500).json({
-        error: `Invalid upload response: ${errorMessage}`,
+        error: `Invalid upload response: ${errorMessage}. The endpoint may not be a valid NIP-95 upload endpoint.`,
       })
     }