feat(ui): overhaul frontend design system and layout

Introduce reusable UI components (AppShell, EntityCard, PageHeader)
Implement Pokemon-themed CSS variables and responsive grids
Refactor all views to adopt the new component structure

frontend/src/components/DetailSection.vue

@@ -0,0 +1,17 @@
+<script setup lang="ts">
+defineProps<{
+  title: string;
+}>();
+</script>
+
+<template>
+  <section class="detail-section">
+    <div class="detail-section__header">
+      <h2>{{ title }}</h2>
+      <slot name="actions"></slot>
+    </div>
+    <div class="detail-section__body">
+      <slot></slot>
+    </div>
+  </section>
+</template>