Update PrepPilot AI branding, add AI powered tag, fix cbt filters and PDF export

src/app/dashboard/ai-coach/cbt-dpp/page.tsx

@@ -15,6 +15,7 @@ export default function AIDailyPracticePage() {
   const [attempts, setAttempts] = useState<any[]>([])
   const [loading, setLoading] = useState(true)
   const [dateFilter, setDateFilter] = useState("")
+  const [diffFilter, setDiffFilter] = useState("Mixed")
   const [selectedPdfDpp, setSelectedPdfDpp] = useState<any | null>(null)
   const [isFullScreen, setIsFullScreen] = useState(false)
   const [copiedShare, setCopiedShare] = useState(false)
@@ -73,6 +74,9 @@ export default function AIDailyPracticePage() {
       const scheduledDate = new Date(dpp.scheduled_date).toISOString().split('T')[0];
       if (scheduledDate !== dateFilter) return false;
     }
+    if (diffFilter !== "Mixed" && dpp.difficulty !== diffFilter) {
+      return false;
+    }
     return true;
   });
 
@@ -100,9 +104,9 @@ export default function AIDailyPracticePage() {
           <div>
             <h1 className="text-3xl md:text-4xl font-black tracking-tight flex items-center gap-3 mb-1">
               <span className="p-2.5 rounded-xl bg-cyan-50 dark:bg-[var(--cyan-ai-10)] border border-cyan-300/60 dark:border-[var(--cyan-ai-20)]">
-                <Sparkles className="text-cyan-700 dark:text-[var(--cyan-ai)]" size={28} />
+                <Zap className="text-cyan-700 dark:text-[var(--cyan-ai)]" size={28} />
               </span>
-              AI Daily Practice (DPP)
+              PrepPilot AI
             </h1>
             <p className="text-muted-foreground text-base font-medium max-w-2xl pl-1">
               Launch AI-generated Daily Practice Problems (DPPs) tailored to your exam pattern and weak areas.