Fix UI hitboxes for CHANGE PLATE and centered Downloads tabs

Add explicit bounds for the CHANGE PLATE control and disable default init (QMF_NODEFAULTINIT) so mouse hit testing matches the rendered label more reliably.

Fine-tune plate button bounds (left/top/right/bottom) to fix vertical and horizontal click-offset issues.

Add QMF_CENTER_JUSTIFY to Downloads tab MTYPE_PTEXT buttons so clickable bounds align with centered tab labels.

Author: Perle2212

engine/code/q3_ui/ui_rally_downloads.c

@@ -888,7 +888,7 @@ static sfxHandle_t DownloadsMenu_Key( int key ) {
 
 static void InitDLTabButton( menutext_s *item, int id, char *label, int x, int y ) {
     item->generic.type     = MTYPE_PTEXT;
-    item->generic.flags    = QMF_PULSEIFFOCUS;
+    item->generic.flags    = QMF_PULSEIFFOCUS | QMF_CENTER_JUSTIFY;
     item->generic.id       = id;
     item->generic.callback = DownloadsMenu_Event;
     item->generic.x        = x;