Implemented phase 1 for the POC prototype in the Website service.

Services/Website/Sources/Library/Internal/Components/SiteLayout.swift

@@ -11,6 +11,9 @@ struct SiteLayout<Content: HTML>: HTML {
     /// The screen's markup, rendered inside the `main` landmark.
     let content: Content
 
+    /// The text the header's search field is filled with, or `nil` to leave it empty.
+    let query: String?
+
     /// The signed-in viewer, or `nil` to render the signed-out header.
     let viewer: Viewer?
 
@@ -19,15 +22,18 @@ struct SiteLayout<Content: HTML>: HTML {
     /// Creates a site layout around a screen's content.
     /// - Parameters:
     ///   - active: the format marked as current, or `nil` (the default) for none.
+    ///   - query: the text the header's search field is filled with, or `nil` (the default) to leave it empty.
     ///   - viewer: the signed-in viewer, or `nil` (the default) to render the signed-out header.
     ///   - content: the screen's markup.
     init(
         active: Format? = nil,
+        query: String? = nil,
         viewer: Viewer? = nil,
         @HTMLBuilder content: () -> Content
     ) {
         self.active = active
         self.content = content()
+        self.query = query
         self.viewer = viewer
     }
 
@@ -42,6 +48,7 @@ struct SiteLayout<Content: HTML>: HTML {
         }
         SiteHeader(
             active: active,
+            query: query,
             viewer: viewer
         )
         main(.id("main")) {