World: add Fullbright visual feature

- New Visuals card with the usual Active + Bind; default off. Unbound+Active
  is always-on, a bind makes it hold-to-brighten.
- Resolve VEngineCvar004 and set mat_fullbright 1/0 through FindVar (slot 12)
  and ConVar::SetValue(int) (slot 11), bypassing the console cheat gate.
  Fall back to ClientCmd if the cvar interface is unavailable.
- Reapply after map loads/respawns and persist [fullbright] active/bind.
- Log on/off; show an error toast if the cvar cannot be applied.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/gui.cpp b/src/gui.cpp
index b02e88a..3f1651d 100644
--- a/src/gui.cpp
+++ b/src/gui.cpp
@@ -372,6 +372,9 @@ static void build_cards() {
              10000, &g_item_max_dist);
   g_ctrls[g_nctrl - 1].zero_unlimited = true;
 
+  add_card(2, &g_fullbright, "Fullbright",
+           "Ignore world lighting so dark areas are visible.", true);
+
   int spread = add_card(3, &g_spread, "Spread control",
                         "Choose a server shot seed that tightens the cone.",
                         true);