Wire Astro content collections to the shared content/ directory

Adds a custom Astro loader that reads the Zola-style TOML frontmatter
(+++ ... +++) directly, so content/ stays the single source of truth
with no YAML conversion step. Defines writings/portfolio/bookshelf/
blogs collections and basic list/detail pages for each, rendering the
seed entries end-to-end (verified in-browser via `pnpm --filter site
dev`). This is the first runnable MVP: home, writings list + post
detail, portfolio, bookshelf (books and papers), and curated blogs
all render from real content/ files.

apps/site/src/layouts/Base.astro

@@ -0,0 +1,62 @@
+---
+interface Props {
+  title: string;
+}
+const { title } = Astro.props;
+---
+
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1" />
+    <title>{title} — andwati.com</title>
+    <style>
+      :root {
+        color-scheme: light dark;
+      }
+      body {
+        font-family: system-ui, sans-serif;
+        max-width: 42rem;
+        margin: 2rem auto;
+        padding: 0 1rem;
+        line-height: 1.6;
+      }
+      nav {
+        display: flex;
+        gap: 1rem;
+        margin-bottom: 2rem;
+        font-size: 0.9rem;
+      }
+      .tag {
+        display: inline-block;
+        font-size: 0.8rem;
+        opacity: 0.7;
+        margin-right: 0.5rem;
+      }
+      .card {
+        margin-bottom: 1.5rem;
+        padding-bottom: 1.5rem;
+        border-bottom: 1px solid color-mix(in srgb, currentColor 15%, transparent);
+      }
+      .card h3 {
+        margin-bottom: 0.25rem;
+      }
+      .meta {
+        font-size: 0.85rem;
+        opacity: 0.7;
+      }
+    </style>
+  </head>
+  <body>
+    <nav>
+      <a href="/">Home</a>
+      <a href="/writings/">Writings</a>
+      <a href="/portfolio/">Portfolio</a>
+      <a href="/bookshelf/">Bookshelf</a>
+      <a href="/blogs/">Blogs</a>
+    </nav>
+    <main>
+      <slot />
+    </main>
+  </body>
+</html>