debug(dashboard): add visible API debug panel to diagnose fetch failures in browser

API endpoints return valid data via curl but browser JS shows 'Failed to load'.
Added verbose logging and a fixed-position debug panel to api.js that displays
the exact error type (JSON parse, API error, or network exception) on-page.
Bumped cache-bust to ?v=4.

static/index.html

@@ -4,7 +4,7 @@
     <meta charset="UTF-8">
     <meta name="viewport" content="width=device-width, initial-scale=1.0">
     <title>LLM Proxy Gateway - Admin Dashboard</title>
-    <link rel="stylesheet" href="/css/dashboard.css?v=3">
+    <link rel="stylesheet" href="/css/dashboard.css?v=4">
     <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/6.4.0/css/all.min.css">
     <link rel="icon" href="img/logo-icon.png" type="image/png" sizes="any">
     <link rel="apple-touch-icon" href="img/logo-icon.png">
@@ -166,19 +166,19 @@
     </div>
 
     <!-- Scripts (cache-busted with version query params) -->
-    <script src="/js/api.js?v=3"></script>
+    <script src="/js/api.js?v=4"></script>
-    <script src="/js/auth.js?v=3"></script>
+    <script src="/js/auth.js?v=4"></script>
-    <script src="/js/dashboard.js?v=3"></script>
+    <script src="/js/dashboard.js?v=4"></script>
-    <script src="/js/websocket.js?v=3"></script>
+    <script src="/js/websocket.js?v=4"></script>
-    <script src="/js/charts.js?v=3"></script>
+    <script src="/js/charts.js?v=4"></script>
-    <script src="/js/pages/overview.js?v=3"></script>
+    <script src="/js/pages/overview.js?v=4"></script>
-    <script src="/js/pages/analytics.js?v=3"></script>
+    <script src="/js/pages/analytics.js?v=4"></script>
-    <script src="/js/pages/costs.js?v=3"></script>
+    <script src="/js/pages/costs.js?v=4"></script>
-    <script src="/js/pages/clients.js?v=3"></script>
+    <script src="/js/pages/clients.js?v=4"></script>
-    <script src="/js/pages/providers.js?v=3"></script>
+    <script src="/js/pages/providers.js?v=4"></script>
-    <script src="/js/pages/models.js?v=3"></script>
+    <script src="/js/pages/models.js?v=4"></script>
-    <script src="/js/pages/monitoring.js?v=3"></script>
+    <script src="/js/pages/monitoring.js?v=4"></script>
-    <script src="/js/pages/settings.js?v=3"></script>
+    <script src="/js/pages/settings.js?v=4"></script>
-    <script src="/js/pages/logs.js?v=3"></script>
+    <script src="/js/pages/logs.js?v=4"></script>
 </body>
 </html>

static/js/api.js

@@ -18,24 +18,62 @@ class ApiClient {
         }
 
         try {
+            console.log(`[API] Fetching ${url}...`);
             const response = await fetch(url, {
                 ...options,
                 headers
             });
 
-            const result = await response.json();
+            console.log(`[API] ${url} → status=${response.status} ok=${response.ok} type=${response.headers.get('content-type')}`);
+
+            const text = await response.text();
+            console.log(`[API] ${url} → body length=${text.length}, first 200 chars:`, text.substring(0, 200));
+
+            let result;
+            try {
+                result = JSON.parse(text);
+            } catch (parseErr) {
+                const msg = `JSON parse failed for ${url}: ${parseErr.message}. Body: ${text.substring(0, 300)}`;
+                console.error(`[API] ${msg}`);
+                this._addDebugEntry(url, 'JSON_PARSE_ERROR', msg);
+                throw new Error(msg);
+            }
 
             if (!response.ok || !result.success) {
+                const msg = `API error for ${url}: ok=${response.ok} success=${result.success} error=${result.error} status=${response.status}`;
+                console.error(`[API] ${msg}`);
+                this._addDebugEntry(url, 'API_ERROR', msg);
                 throw new Error(result.error || `HTTP error! status: ${response.status}`);
             }
 
+            console.log(`[API] ${url} → SUCCESS, data keys:`, result.data ? Object.keys(result.data) : 'null');
             return result.data;
         } catch (error) {
-            console.error(`API Request failed (${path}):`, error);
+            console.error(`[API] Request failed (${path}):`, error);
+            this._addDebugEntry(url, 'EXCEPTION', error.message);
             throw error;
         }
     }
 
+    // Visible on-page debug panel for diagnosing fetch failures
+    _addDebugEntry(url, status, detail) {
+        let panel = document.getElementById('api-debug-panel');
+        if (!panel) {
+            panel = document.createElement('div');
+            panel.id = 'api-debug-panel';
+            panel.style.cssText = 'position:fixed;bottom:0;left:0;right:0;max-height:200px;overflow-y:auto;background:#1d2021;color:#fbf1c7;font-family:monospace;font-size:11px;padding:8px;z-index:99999;border-top:2px solid #cc241d;';
+            const title = document.createElement('div');
+            title.style.cssText = 'font-weight:bold;margin-bottom:4px;color:#fb4934;';
+            title.textContent = 'API Debug Panel (remove after fixing)';
+            panel.appendChild(title);
+            document.body.appendChild(panel);
+        }
+        const entry = document.createElement('div');
+        entry.style.cssText = 'margin:2px 0;padding:2px 4px;background:#282828;border-left:3px solid ' + (status === 'EXCEPTION' ? '#fb4934' : '#fabd2f') + ';';
+        entry.textContent = `[${status}] ${url}: ${detail}`;
+        panel.appendChild(entry);
+    }
+
     async get(path) {
         return this.request(path, { method: 'GET' });
     }