fix(blog): make RSS feed discoverable site-wide

The RSS autodiscovery <link> only lived on the blog index and post
pages, so a feed reader pointed at the root domain (postguard.eu) or
any other marketing page found no feed. Move the autodiscovery link
into the marketing layout head so every public page advertises the
feed, and drop the now-redundant per-page duplicate on /blog/.

Refs encryption4all/postguard-business#58

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dobby-coder[bot]

src/routes/(marketing)/+layout.svelte

@@ -19,6 +19,17 @@
     })
 </script>
 
+<svelte:head>
+    <!-- Site-wide RSS autodiscovery so feed readers pointed at any marketing
+         page (including the root domain) find the blog feed. -->
+    <link
+        rel="alternate"
+        type="application/rss+xml"
+        title="PostGuard Blog RSS Feed"
+        href="/blog/rss.xml"
+    />
+</svelte:head>
+
 {#if !$isLoading}
     <a class="sr-only sr-only-focusable skip-link" href="#main-content">
         {$_('common.skipToMain', { default: 'Skip to main content' })}

src/routes/(marketing)/blog/+page.svelte

@@ -54,15 +54,6 @@
     jsonLd={blogJsonLd}
 />
 
-<svelte:head>
-    <link
-        rel="alternate"
-        type="application/rss+xml"
-        title="PostGuard Blog RSS Feed"
-        href="/blog/rss.xml"
-    />
-</svelte:head>
-
 <div class="blog-index">
     <div class="header">
         <h1>Blog</h1>