feat: GitHub stars badge on open-source projects via shields.io

- New openSource field in project model distinguishes OS from private
- shields.io badge shows live star count for open-source projects
- SoundInbox marked as NOT open source (no badge, no stars)
- Immich Photo Manager, Cmux Resurrect, Gotify Commander, CDC Starter Kit → open source
- Stars badge hidden in print view

internal/templates/template.go

@@ -68,6 +68,15 @@ func (m *Manager) loadTemplatesLocked() error {
 		"replaceDrolosoft": func(s string) string {
 			return strings.Replace(s, "drolosoft", `<a href="https://drolosoft.com" target="_blank" rel="noopener noreferrer">drolosoft</a>`, 1)
 		},
+		// githubRepo extracts "owner/repo" from a GitHub URL
+		"githubRepo": func(url string) string {
+			url = strings.TrimSuffix(url, "/")
+			parts := strings.Split(url, "github.com/")
+			if len(parts) == 2 {
+				return parts[1]
+			}
+			return ""
+		},
 		// dict creates a map from key-value pairs for passing to sub-templates
 		"dict": func(values ...interface{}) (map[string]interface{}, error) {
 			if len(values)%2 != 0 {