feat: simplify materials library and use real metadata

pages/ManageCourses.tsx

@@ -395,13 +395,23 @@ const CourseModal: React.FC<{
       alert("Informe o nome do arquivo para download.");
       return;
     }
+    if (!newAttUrl.trim()) {
+      alert('Informe a URL do material.');
+      return;
+    }
+    try {
+      new URL(newAttUrl.trim());
+    } catch {
+      alert('Informe uma URL válida para o material.');
+      return;
+    }
 
     const att: Attachment = {
       id: `att-${Date.now()}`,
       name: newAttName.trim(),
       type: newAttType,
-      url: newAttUrl.trim() || '#',
-      size: newAttSize.trim() || '1.5 MB',
+      url: newAttUrl.trim(),
+      size: newAttSize.trim() || undefined,
       description: newAttDesc.trim()
     };
 
@@ -679,12 +689,21 @@ const CourseModal: React.FC<{
                 </select>
               </div>
 
+              <div className="flex gap-2">
+                <input
+                  type="url"
+                  value={newAttUrl}
+                  onChange={(e) => setNewAttUrl(e.target.value)}
+                  placeholder="URL do material (https://...)"
+                  className="min-w-0 flex-1 bg-white/5 border border-white/10 rounded-lg px-2.5 py-1.5 text-xs text-white focus:outline-none"
+                />
+              </div>
               <div className="flex gap-2">
                 <input 
                   type="text" 
                   value={newAttSize}
                   onChange={(e) => setNewAttSize(e.target.value)}
-                  placeholder="Ex.: 2,4 MB"
+                  placeholder="Tamanho (opcional)"
                   className="w-28 bg-white/5 border border-white/10 rounded-lg px-2.5 py-1.5 text-xs text-white focus:outline-none"
                 />
                 <input