chore: update demo social links for tech-savvy POD seller persona

Select platforms typical for a quirky, nerdy POD seller:
- Instagram: primary visual portfolio
- Bluesky: nerdy Twitter alternative
- Mastodon: federated/open web presence
- Ko-fi: indie tips and commissions
- GitHub: open source projects

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/simpleshop_theme_web/components/page_templates/contact.html.heex

@@ -28,7 +28,13 @@
 
         <.newsletter_card />
 
-        <.social_links_card />
+        <.social_links_card links={[
+          %{platform: :instagram, url: "#", label: "Instagram"},
+          %{platform: :bluesky, url: "#", label: "Bluesky"},
+          %{platform: :mastodon, url: "#", label: "Mastodon"},
+          %{platform: :kofi, url: "#", label: "Ko-fi"},
+          %{platform: :github, url: "#", label: "GitHub"}
+        ]} />
       </div>
     </div>
   </main>