feat(missions): keep the viewed flag of missions in progress, as the client

The client keeps a viewed flag on each mission in progress, sets it with
SetMissionViewed (LWOMissionComponent::msgSetMissionViewed 0x00bec7a0, only
for missions in progress) and reads it as v on the mission's <cur> entry
(LWOMissionComponent::LoadFromSaveData 0x00d171c0, a number or "true", false
when missing). The server now handles SetMissionViewed, saves v="1" for a
viewed mission in progress and loads it back; saves without v load as not
viewed.

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

dGame/dGameMessages/GameMessageHandler.cpp

@@ -82,6 +82,7 @@ namespace {
 		{ SET_FLAG, []() { return std::make_unique<SetFlag>(); } },
 		{ HAS_BEEN_COLLECTED, []() { return std::make_unique<HasBeenCollected>(); } },
 		{ SET_MISSION_TYPE_STATE, []() { return std::make_unique<SetMissionTypeState>(); } },
+		{ SET_MISSION_VIEWED, []() { return std::make_unique<SetMissionViewed>(); } },
 
 		// Effects, emotes, cinematics, UI
 		{ PLAY_EMOTE, []() { return std::make_unique<PlayEmote>(); } },