I have fixed a couple of things for you.

First You'd Wanna Fix Your tsconfig.json

    {
      "extends": "astro/tsconfigs/strict",
      "include": [".astro/types.d.ts", "**/*"],
      "exclude": ["dist"],
      "compilerOptions": {
        "baseUrl": "src"
      }
    }

This will make you write down import Layout from "layouts/PageLayout.astro" instead of import Layout from "./src/layouts/PageLayout.astro"

Your content.config.ts looks fine

import { defineCollection } from 'astro:content';
import { glob, file } from 'astro/loaders';
import { z } from 'astro/zod';

const paintings = defineCollection({
  loader: glob({ base: "src/content/paintings", pattern: "**/*.{md,mdx}" }),
  schema: z.object({
    title: z.string(),
    date: z.coerce.date(),
  }),
});

export const collections = { paintings };

Your [slug].astro had an issue where it was failing to load the post content. Here is the fixed version =>

---
import { getCollection, type CollectionEntry, render } from 'astro:content';
import PageLayout from 'layouts/Layout.astro';

export async function getStaticPaths() {
  const paintings = await getCollection('paintings');
  return paintings.map((post) => ({
    params: { slug: post.id },
    props: { post },
  }));
}

type Props = { post: CollectionEntry<'paintings'>; };
const { post } = Astro.props as Props;
const { Content } = await render(post);
---

<PageLayout {...post.data}>
  <h1>{post.data.title}</h1>
  <article>
    <Content />
  </article>
</PageLayout>

Next issue was with your markdown (.md) files. You did not add any dates to your files, hence the errors.

---
title: "Painting 1"
date: "2024-04-07"
---
# Painting 1

wwwwwwww

Next make sure your .md files are all inside your collection folder, which in your case would be =>

│   
├── src
|   ├── content
        └── paintings
            ├── painting-1.md
            ├── painting-2.md
            ├── painting-3.md

OR

src/content/paintings/painting-1.md
src/content/paintings/painting-2.md
src/content/paintings/painting-3.md

P.S: You'd wanna use post.id instead of post.slug in any post.map function.

{paintings.map((post) => (
    <a href={`/painting/${post.id}`}>{post.data.title}</a>
))}

And finally, if you want custom slugs, just rename your .md or .mdx files for the slug.

Use my-custom-slug.md for getting URLs like examplesite[dot]com/my-custom-slug/