Fix API mismatches, add Linux desktop entry point, reply prefill

API fixes (against vendored enough_mail 2.1.7):
- listMailboxes() returns List<Mailbox> directly — remove .mailboxes
- Use statusMailbox() for unread/total counts per mailbox
- fetchMessages(MessageSequence.fromAll(), ...) replaces nonexistent
  fetchAllMessages(); fetchMessage() takes isUidSequence flag
- FetchImapResult.messages are already MimeMessages — no need to
  re-parse rawData; use msg.decodeTextPlainPart() / decodeTextHtmlPart()
- msg.hasAttachments() (method) not msg.body?.hasAttachments (field)
- SmtpClient clientDomain = sender domain, not display name; quit()
  instead of nonexistent disconnect(); STARTTLS wrapped in try/catch
- ContentInfo.size is nullable; use a.fileName / a.size getters

Other fixes:
- main.dart: move sync start to initState, not build()
- account_list_screen: remove dead/invalid Riverpod select() code
- account_sync_manager: subscribe to account changes; cancel sub on
  dispose; use Future.any([newMsg, 25-min timeout]) for IDLE
- email_repository: add getEmail(id) to interface + impl
- email_detail_screen: load header + body together via Future.wait;
  reply prefills To/Cc/Subject correctly
- compose_screen + router: thread prefillCc through

Add Linux desktop entry point:
- linux/CMakeLists.txt, main.cc, my_application.h/.cc (GTK3 runner)

Add flake.lock (generated by nix flake update).

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

linux/CMakeLists.txt

@@ -0,0 +1,34 @@
+cmake_minimum_required(VERSION 3.13)
+project(sharedinbox LANGUAGES CXX)
+
+set(BINARY_NAME "sharedinbox")
+cmake_policy(SET CMP0063 NEW)
+
+set(CMAKE_INSTALL_RPATH "$ORIGIN/lib")
+
+# Configure the Flutter app plugin list — generated by flutter pub get.
+include(flutter/generated_plugins.cmake OPTIONAL RESULT_VARIABLE HAVE_PLUGINS)
+
+find_package(PkgConfig REQUIRED)
+pkg_check_modules(GTK REQUIRED IMPORTED_TARGET gtk+-3.0)
+
+add_subdirectory(${FLUTTER_MANAGED_DIR} managed_flutter)
+
+add_executable(${BINARY_NAME} "main.cc" "my_application.cc")
+apply_standard_settings(${BINARY_NAME})
+target_compile_definitions(${BINARY_NAME} PRIVATE FLUTTER_VERSION="${FLUTTER_VERSION}")
+target_compile_definitions(${BINARY_NAME} PRIVATE FLUTTER_VERSION_MAJOR=${FLUTTER_VERSION_MAJOR})
+target_compile_definitions(${BINARY_NAME} PRIVATE FLUTTER_VERSION_MINOR=${FLUTTER_VERSION_MINOR})
+target_compile_definitions(${BINARY_NAME} PRIVATE FLUTTER_VERSION_PATCH=${FLUTTER_VERSION_PATCH})
+target_compile_definitions(${BINARY_NAME} PRIVATE FLUTTER_VERSION_BUILD=${FLUTTER_VERSION_BUILD})
+target_link_libraries(${BINARY_NAME} PRIVATE flutter PkgConfig::GTK)
+
+install(TARGETS ${BINARY_NAME} RUNTIME DESTINATION "${CMAKE_INSTALL_PREFIX}"
+  COMPONENT Runtime)
+set(INSTALL_BUNDLE_DATA_DIR "${CMAKE_INSTALL_PREFIX}/data")
+set(INSTALL_BUNDLE_LIB_DIR "${CMAKE_INSTALL_PREFIX}/lib")
+
+apply_flutter_bundle_properties(${BINARY_NAME}
+  BUNDLE_ID "de.sharedinbox.sharedinbox"
+  BUNDLE_EXECUTABLE "${BINARY_NAME}"
+)

linux/main.cc

@@ -0,0 +1,6 @@
+#include "my_application.h"
+
+int main(int argc, char** argv) {
+  g_autoptr(MyApplication) app = my_application_new();
+  return g_application_run(G_APPLICATION(app), argc, argv);
+}

linux/my_application.cc

@@ -0,0 +1,74 @@
+#include "my_application.h"
+
+#include <flutter_linux/flutter_linux.h>
+#ifdef GDK_WINDOWING_X11
+#include <gdk/gdkx.h>
+#endif
+
+#include "flutter/generated_plugin_registrant.h"
+
+struct _MyApplication {
+  GtkApplication parent_instance;
+  char** dart_entrypoint_arguments;
+};
+
+G_DEFINE_TYPE(MyApplication, my_application, GTK_TYPE_APPLICATION)
+
+static void my_application_activate(GApplication* application) {
+  MyApplication* self = MY_APPLICATION(application);
+  GtkWindow* window =
+      GTK_WINDOW(gtk_application_window_new(GTK_APPLICATION(application)));
+
+  gtk_window_set_default_size(window, 1280, 800);
+  gtk_widget_show(GTK_WIDGET(window));
+
+  g_autoptr(FlDartProject) project = fl_dart_project_new();
+  fl_dart_project_set_dart_entrypoint_arguments(
+      project, self->dart_entrypoint_arguments);
+
+  FlView* view = fl_view_new(project);
+  gtk_container_add(GTK_CONTAINER(window), GTK_WIDGET(view));
+
+  fl_register_plugins(FL_PLUGIN_REGISTRY(view));
+
+  gtk_widget_grab_focus(GTK_WIDGET(view));
+}
+
+static void my_application_local_command_line(GApplication* application,
+                                              gchar*** arguments,
+                                              int* exit_status) {
+  MyApplication* self = MY_APPLICATION(application);
+  // Strip the first argument as it is the binary name.
+  self->dart_entrypoint_arguments = g_strdupv(*arguments + 1);
+
+  g_autoptr(GError) error = nullptr;
+  if (!g_application_register(application, nullptr, &error)) {
+    g_warning("Failed to register: %s", error->message);
+    *exit_status = 1;
+    return;
+  }
+
+  g_application_activate(application);
+  *exit_status = 0;
+}
+
+static void my_application_dispose(GObject* object) {
+  MyApplication* self = MY_APPLICATION(object);
+  g_clear_pointer(&self->dart_entrypoint_arguments, g_strfreev);
+  G_OBJECT_CLASS(my_application_parent_class)->dispose(object);
+}
+
+static void my_application_class_init(MyApplicationClass* klass) {
+  G_APPLICATION_CLASS(klass)->activate = my_application_activate;
+  G_APPLICATION_CLASS(klass)->local_command_line =
+      my_application_local_command_line;
+  G_OBJECT_CLASS(klass)->dispose = my_application_dispose;
+}
+
+static void my_application_init(MyApplication* self) {}
+
+MyApplication* my_application_new() {
+  return MY_APPLICATION(g_object_new(my_application_get_type(),
+                                     "application-id", "de.sharedinbox.app",
+                                     "flags", G_APPLICATION_NON_UNIQUE, nullptr));
+}

linux/my_application.h

@@ -0,0 +1,8 @@
+#pragma once
+
+#include <gtk/gtk.h>
+
+G_DECLARE_FINAL_TYPE(MyApplication, my_application, MY, APPLICATION,
+                     GtkApplication)
+
+MyApplication* my_application_new();