add api client and auth store

apiFetch wrapper sends JSON with credentials, parses error
shapes off the backend's {error: '...'} responses, and exposes
typed helpers (apiGet, apiPost, apiDelete). Auth store now
drives the real /signup -> /confirm-email -> /login -> /me ->
/logout flow. Vite dev proxy points /api at the backend on
:8000.

frontend/blog_portal/src/stores/auth.ts

@@ -1,5 +1,6 @@
 import { defineStore } from "pinia";
-import { ref } from "vue";
+import { computed, ref } from "vue";
+import { apiGet, apiPost } from "@/api/client";
 
 export interface AuthUser {
   id: number;
@@ -8,18 +9,78 @@ export interface AuthUser {
   isAdmin: boolean;
 }
 
+interface MePayload {
+  user: AuthUser;
+}
+
 export const useAuthStore = defineStore("auth", () => {
   const user = ref<AuthUser | null>(null);
   const checked = ref(false);
   const error = ref<string | null>(null);
+  const signupCompleted = ref(false);
+
+  const isAuthenticated = computed(() => user.value !== null);
+  const isAdmin = computed(() => user.value?.isAdmin === true);
 
   async function fetchMe(): Promise<boolean> {
+    const result = await apiGet<MePayload>("/me");
     checked.value = true;
+    if (result.ok && result.data) {
+      user.value = result.data.user;
+      return true;
+    }
+    user.value = null;
     return false;
   }
 
+  async function signup(email: string, displayName: string): Promise<boolean> {
+    error.value = null;
+    const result = await apiPost<null>("/signup", {
+      email,
+      displayName,
+    });
+    if (!result.ok) {
+      error.value = result.error;
+      return false;
+    }
+    signupCompleted.value = true;
+    return true;
+  }
+
+  async function confirmEmail(token: string, password: string): Promise<boolean> {
+    error.value = null;
+    const result = await apiPost<null>("/confirm-email", {
+      token,
+      password,
+    });
+    if (!result.ok) {
+      error.value = result.error;
+      return false;
+    }
+    return true;
+  }
+
+  async function login(email: string, password: string): Promise<boolean> {
+    error.value = null;
+    const result = await apiPost<MePayload>("/login", {
+      email,
+      password,
+    });
+    if (!result.ok) {
+      error.value = result.error;
+      return false;
+    }
+    if (result.data) {
+      user.value = result.data.user;
+      checked.value = true;
+    }
+    return true;
+  }
+
   async function logout(): Promise<void> {
+    await apiPost<null>("/logout", {});
     user.value = null;
+    signupCompleted.value = false;
   }
 
   function clearError(): void {
@@ -30,7 +91,13 @@ export const useAuthStore = defineStore("auth", () => {
     user,
     checked,
     error,
+    signupCompleted,
+    isAuthenticated,
+    isAdmin,
     fetchMe,
+    signup,
+    confirmEmail,
+    login,
     logout,
     clearError,
   };