feat: add home URL metadata support for navigation buttons

fixes googlecodelabs#103 and googlecodelabs#535

Further discussion:
https://groups.google.com/g/codelab-authors/c/UX-tcdc0P20/m/Arl0UdbBBAAJ
https://groups.google.com/g/codelab-authors/c/x0CwV2yg6tA/m/WjRqNCNpBwAJ

Author: Bit-Blazer

claat/parser/md/parse.go

@@ -57,6 +57,7 @@ const (
 	MetaTags                = "tags"
 	MetaSource              = "source"
 	MetaDuration            = "duration"
+	MetaHomeURL             = "home_url"
 )
 
 const (
@@ -437,6 +438,9 @@ func addMetadataToCodelab(m map[string]string, c *types.Codelab, opts parser.Opt
 		case MetaFeedbackLink:
 			// Directly assign the feedback link to the codelab field.
 			c.Feedback = v
+		case MetaHomeURL:
+			// Directly assign the home URL to the codelab field.
+			c.HomeURL = v
 		case MetaAnalyticsAccount:
 			// Directly assign the GA id to the codelab field.
 			c.GA = v

claat/render/template.html

@@ -59,6 +59,16 @@
   <script>
     hljs.highlightAll();
     hljs.addPlugin(new CopyButtonPlugin({ autohide: false}));
+    {{if .Meta.HomeURL}}
+    // Override home URL with metadata value
+    document.addEventListener('DOMContentLoaded', function() {
+      const homeUrl = "{{.Meta.HomeURL}}";
+      const arrowBack = document.getElementById('arrow-back');
+      const doneBtn = document.getElementById('done');
+      if (arrowBack) arrowBack.href = homeUrl;
+      if (doneBtn) doneBtn.href = homeUrl;
+    });
+    {{end}}
   </script>
 
 </body>

claat/types/codelab.go

@@ -36,6 +36,7 @@ type Meta struct {
 	Feedback   string            `json:"feedback,omitempty"` // Issues and bugs are sent here
 	GA         string            `json:"ga,omitempty"`       // Codelab-specific GA tracking ID
 	GA4        string            `json:"ga4,omitempty"`      // Codelab-specific GA4 tracking ID
+	HomeURL    string            `json:"home_url,omitempty"` // Optional home URL for back/done buttons
 	Extra      map[string]string `json:"extra,omitempty"`    // Extra metadata specified in pass_metadata
 
 	URL string `json:"url"` // Legacy ID; TODO: remove