Add About OOTT page with version, links, and legal notices

Implements the /about route with app description, version info,
clickable links to the source repo and AGPL-3.0 license, and
third-party copyright notices. Adds url_launcher for cross-platform
link handling.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/linux/flutter/generated_plugin_registrant.cc

@@ -7,9 +7,13 @@
 #include "generated_plugin_registrant.h"
 
 #include <encrypter/encrypter_plugin.h>
+#include <url_launcher_linux/url_launcher_plugin.h>
 
 void fl_register_plugins(FlPluginRegistry* registry) {
   g_autoptr(FlPluginRegistrar) encrypter_registrar =
       fl_plugin_registry_get_registrar_for_plugin(registry, "EncrypterPlugin");
   encrypter_plugin_register_with_registrar(encrypter_registrar);
+  g_autoptr(FlPluginRegistrar) url_launcher_linux_registrar =
+      fl_plugin_registry_get_registrar_for_plugin(registry, "UrlLauncherPlugin");
+  url_launcher_plugin_register_with_registrar(url_launcher_linux_registrar);
 }

frontend/linux/flutter/generated_plugins.cmake

@@ -4,6 +4,7 @@
 
 list(APPEND FLUTTER_PLUGIN_LIST
   encrypter
+  url_launcher_linux
 )
 
 list(APPEND FLUTTER_FFI_PLUGIN_LIST