feat: Add JWT authentication to backend, configure frontend API client, fix streak layout

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

@@ -1,5 +1,6 @@
 "use client";
 
+import { fetchWithAuth } from "@/lib/apiClient";
 import { useEffect, useState } from "react"
 import { MistakeHeatmap } from "@/components/AI/MistakeHeatmap"
 import { StudyPlanCard } from "@/components/AI/StudyPlanCard"
@@ -39,7 +40,7 @@ export default function AICoachDashboard() {
 
   useEffect(() => {
     if (!userLoading && userId) {
-      fetch(`/api/users/${userId}/stats`)
+      fetchWithAuth(`/api/users/${userId}/stats`)
         .then(res => res.ok ? res.json() : null)
         .then(data => setStats(data))
         .catch(() => setStats(null))