feat(header): keep internal social-nav links in same tab

Social-link entries with internal URLs (starting with /) used to open in a
new tab via target=_blank, which is wrong for internal navigation like the
new /kalender entry.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lib/components/Header.svelte

@@ -137,10 +137,11 @@
             {/each}
             <div class="w-1 h-4"></div>
             {#each socialLinks as social}
+                {@const isInternal = social.href.startsWith("/")}
                 <a
                     href={social.href}
-                    target="_blank"
-                    rel="noopener noreferrer"
+                    target={isInternal ? undefined : "_blank"}
+                    rel={isInternal ? undefined : "noopener noreferrer"}
                     class="p-1.5 -m-1.5 rounded-xs transition-colors flex items-center justify-center"
                     aria-label={social.label || t(T.header_social_aria)}
                 >
@@ -260,10 +261,13 @@
                         class="flex flex-wrap gap-2 pt-2 mt-2 border-t border-stein-0/10"
                     >
                         {#each socialLinks as social}
+                            {@const isInternal = social.href.startsWith("/")}
                             <a
                                 href={social.href}
-                                target="_blank"
-                                rel="noopener noreferrer"
+                                target={isInternal ? undefined : "_blank"}
+                                rel={isInternal
+                                    ? undefined
+                                    : "noopener noreferrer"}
                                 class="inline-flex p-2 text-stein-100 hover:text-stein-0 hover:bg-stein-0/10 rounded-xs transition-colors"
                                 aria-label={social.label ||
                                     t(T.header_social_aria)}