Fix missing slash in release notes URL generation (#946)

Use url.JoinPath() to properly construct the release notes URL instead
of string concatenation, which was missing a leading slash and could
fail with URLs that have trailing slashes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: wiggin77

build/manifest/main.go

@@ -6,6 +6,7 @@ package main
 import (
 	"encoding/json"
 	"fmt"
+	"net/url"
 	"os"
 	"strings"
 
@@ -138,7 +139,10 @@ func findManifest() (*model.Manifest, error) {
 
 	// If no release notes specified, generate one from the latest tag, if present.
 	if manifest.ReleaseNotesURL == "" && BuildTagLatest != "" {
-		manifest.ReleaseNotesURL = manifest.HomepageURL + "releases/tag/" + BuildTagLatest
+		manifest.ReleaseNotesURL, err = url.JoinPath(manifest.HomepageURL, "releases", "tag", BuildTagLatest)
+		if err != nil {
+			return nil, errors.Wrap(err, "failed to generate release notes URL")
+		}
 	}
 
 	return &manifest, nil