feat(website): joins cast on the querying-with-joins guide

Fourth cast: build a minimal two-table schema with rows, switch to advanced
mode (`mode advanced`), and run a join pairing each book with its author —
shows the mode switch + SQL + multi-table result, motion that complements the
guide's static examples. Convert the guide to .mdx and embed above the intro.

Recorded via `pnpm casts`; build clean (25 pages).

website/src/content/docs/guides/querying-with-joins.mdx

@@ -5,12 +5,16 @@ sidebar:
   order: 2
 ---
 
+import Demo from '../../../components/Demo.astro';
+
 [Build the library](/guides/build-the-library/) left you with four related
 tables. So far each `show data` looks at one table at a time — but the
 interesting questions span tables: *who wrote this book? who has borrowed it?*
 Answering those means a **join**: matching rows from one table against related
 rows in another.
 
+<Demo src="/casts/joins.cast" title="Switch to advanced mode, then join books to their authors." />
+
 Joins are part of SQL, so this guide is in **advanced mode**. Switch to it with
 the `mode` command: