Fix blog post date filter and add generated OG images

- Fix Nunjucks filter name from toISOString to toIsoString in blog.njk
- Add generated Open Graph preview images for all 8 blog posts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dougaitken

src/assets/og-images/advice-from-a-constant-job-seeker-preview.jpeg

@@ -23,7 +23,7 @@ description: 'Remote work advice, tips, and updates from the Remote In Tech comm
                 <a href="{{ post.url }}">{{ post.data.title }}</a>
               </h2>
               <p class="meta">
-                <time datetime="{{ post.date | toISOString }}">
+                <time datetime="{{ post.date | toIsoString }}">
                   {{ post.date | formatDate }}
                 </time>
                 {% if post.data.author %}