feat(layout): add PageShell, ColumnsLayout, PageSection primitives

Introduces three new layout primitives (no consumers yet). Adds
--page-pad-x/y/collapse CSS tokens to :root and .dark. Updates
PageHeader to read padding from tokens. Removes ignored size="large"
props from welcome and series pages. Fixes stray markdown in SidebarLayout.

app/components/PageShell.vue

@@ -0,0 +1,24 @@
+<template>
+  <component :is="as" class="page-shell">
+    <PageHeader v-if="title" :title="title" :subtitle="subtitle" />
+    <slot />
+  </component>
+</template>
+
+<script setup>
+defineProps({
+  title: { type: String, default: '' },
+  subtitle: { type: String, default: '' },
+  as: { type: String, default: 'div' },
+})
+</script>
+
+<style scoped>
+.page-shell {
+  flex: 1;
+  display: flex;
+  flex-direction: column;
+  min-height: 0;
+  width: 100%;
+}
+</style>