Implemented phase 3 for the POC prototype in the Website service.

Services/Website/Tests/Library/Cases/Internal/Components/SiteHeaderTests.swift

@@ -25,6 +25,7 @@ struct SiteHeaderTests {
     func `renders the signed-in actions for a viewer`() {
         let html = SiteHeader(viewer: .init(
             displayName: "Mara Visser",
+            shopSlug: "kestrel-row-comics",
             newWantMatches: 2,
             unreadMessages: 3
         )).render()
@@ -38,6 +39,11 @@ struct SiteHeaderTests {
         #expect(!html.contains(#"href="/sign-in""#))
     }
 
+    @Test
+    func `links the shop of a seller alone`() {
+        #expect(!SiteHeader(viewer: .init(displayName: "Mara")).render().contains(#"href="/sell""#))
+    }
+
     @Test
     func `omits a badge that counts nothing`() {
         let html = SiteHeader(viewer: .init(displayName: "Mara")).render()