books: import zod as a namespace so tests pass under the Bun runtime in CI

The Docker build runs vitest via bun (no Node in oven/bun), where the named
`{ z }` import from zod 4 resolved to undefined; the rest of the repo already
uses `import * as z from 'zod'`.

src/lib/books/progress.ts

@@ -1,5 +1,5 @@
 // Validation and maths for the reader's progress API (pure, unit tested).
-import { z } from 'zod';
+import * as z from 'zod';
 
 export const progressPayload = z.object({
 	fileId: z.number().int().positive().nullish(),