feat(branding): rename fork to Nightcap and add AppImage release workflow

Show Nightcap as the app name in the desktop entry, window title,
dialogs, and metainfo. Binary, config paths, and desktop ID keep the
mocktail name. Publish Nightcap-x86_64.AppImage to GitHub Releases on
v* tags. README points at the fork's releases instead of upstream
packages.

src/ui/mocktail_failure_dialog.cc

@@ -1,3 +1,4 @@
+// Modified by vii from komaruworld/mocktail. See README "About this fork".
 #include <adwaita.h>
 #include <glib-unix.h>
 #include <sys/socket.h>
@@ -153,7 +154,7 @@ void InstallProgressStyle(GdkDisplay* display) {
 }
 
 bool InitializeUi(UiStyle style) {
-  g_set_application_name("Mocktail");
+  g_set_application_name("Nightcap");
   if (!gtk_init_check()) {
     return false;
   }
@@ -229,7 +230,7 @@ void PresentProgressWindow(ProgressState* state, std::string_view message) {
   GtkWidget* window_widget = adw_window_new();
   state->window = GTK_WINDOW(g_object_ref_sink(window_widget));
   gtk_widget_add_css_class(window_widget, "mocktail-update-progress");
-  gtk_window_set_title(state->window, "Mocktail");
+  gtk_window_set_title(state->window, "Nightcap");
   gtk_window_set_default_size(state->window, 470, 640);
 
   GtkWidget* toolbar_widget = adw_toolbar_view_new();