Give every dashboard page its own URL and move add-forms into dialogs

Pages are served at /resources, /sites, /domains, /mail-egress, /alerts, /access, /logs,
/apps, /account and /server/*, with working links, reloads and back button. Mail egress,
alerts, access, users, organizations, SSO and DKIM follow the same header-button and dialog
pattern as the other pages.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

server.go

@@ -21,11 +21,22 @@ func page(html []byte) http.HandlerFunc {
 	}
 }
 
+// dashboardPaths are the addresses the dashboard shows pages at. They all serve the same app,
+// which reads the path to decide what to show. Keep in step with TAB_PATH in web/index.html.
+var dashboardPaths = map[string]bool{
+	"/": true, "/apps": true, "/resources": true, "/sites": true, "/domains": true, "/mail-egress": true,
+	"/alerts": true, "/access": true, "/logs": true, "/account": true,
+	"/server/edges": true, "/server/organizations": true, "/server/users": true, "/server/sso": true,
+	"/server/settings": true, "/server/alerts": true, "/server/audit": true,
+}
+
+func isDashboardPath(p string) bool { return dashboardPaths[p] }
+
 func (s *Server) Handler() http.Handler {
 	mux := http.NewServeMux()
 	index := page(indexHTML)
 	mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
-		if r.URL.Path != "/" {
+		if !isDashboardPath(r.URL.Path) {
 			http.NotFound(w, r)
 			return
 		}