scaffold blog_portal vue 3 + pinia + cypress frontend

Mirrors youngstartup/frontend/startups_portal scaffolding:
Vite, Vue 3 (composition API + script setup), TypeScript strict,
Pinia, Vue Router 5, oxlint + eslint + oxfmt, and Cypress with
db:reset / db:seed tasks. Views and the auth store are stubs
filled in by the next branches; routes and the header chrome
are wired so the build passes.

frontend/blog_portal/src/views/SignupPage.vue

@@ -0,0 +1,7 @@
+<script setup lang="ts">
+// SignupPage - filled in by a later branch.
+</script>
+
+<template>
+  <section><h1>SignupPage</h1></section>
+</template>