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/vite.config.ts

@@ -7,14 +7,18 @@ import vueDevTools from 'vite-plugin-vue-devtools'
 
 // https://vite.dev/config/
 export default defineConfig({
-  plugins: [
-    vue(),
-    vueJsx(),
-    vueDevTools(),
-  ],
+  plugins: [vue(), vueJsx(), vueDevTools()],
   resolve: {
     alias: {
-      '@': fileURLToPath(new URL('./src', import.meta.url))
+      '@': fileURLToPath(new URL('./src', import.meta.url)),
+    },
+  },
+  server: {
+    proxy: {
+      '/api': {
+        target: 'http://localhost:8000',
+        changeOrigin: true,
+      },
     },
   },
 })