refactor(v2): migrate has_snap_welcome_been_shown to v2

Add v2_has_snap_welcome_been_shown as a thin wrapper over the
existing UpdatesStore::has_snap_welcome_been_shown(). Update the
frontend invoke in updatesService.ts to use the v2 name. Legacy
command remains for now.

Audit: legacy_callsites 15 -> 14.

cc

src-tauri/src/commands_v2/legacy_compat.rs

@@ -1493,6 +1493,13 @@ pub fn v2_mark_snap_welcome_shown(
     crate::updates::mark_snap_welcome_shown(state)
 }
 
+#[tauri::command]
+pub fn v2_has_snap_welcome_been_shown(
+    state: State<'_, crate::updates::UpdatesState>,
+) -> Result<bool, String> {
+    crate::updates::has_snap_welcome_been_shown(state)
+}
+
 #[tauri::command]
 pub async fn v2_detect_legacy_cached_files(
     cache_state: State<'_, OfflineCacheState>,

src-tauri/src/lib.rs

@@ -1579,6 +1579,7 @@ pub fn run() {
             commands_v2::v2_is_running_in_flatpak,
             commands_v2::v2_is_running_in_snap,
             commands_v2::v2_mark_snap_welcome_shown,
+            commands_v2::v2_has_snap_welcome_been_shown,
             commands_v2::v2_detect_legacy_cached_files,
             commands_v2::v2_reco_log_event,
             commands_v2::v2_reco_train_scores,