feat(bookmarks): add bookmark background art and suppress vanilla bookmarks

Add background DDS art for the "End of an Empire" bookmark and
reference it in the bookmark definition. Add vanilla bookmark
suppression file to hide all base-game bookmarks since this is a
total conversion.

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

lords_of_ash/common/bookmarks/bookmarks/00_mistborn_bookmarks.txt

@@ -7,6 +7,7 @@ bm_end_of_an_empire = {
 	start_date = "1020.1.1"
 	is_playable = yes
 	recommended = yes
+	background = "gfx/interface/bookmarks/bm_end_of_an_empire.dds"
 
 	weight = {
 		value = 100

lords_of_ash/common/bookmarks/bookmarks/00_vanilla_suppress.txt

@@ -0,0 +1,23 @@
+############################################
+# Suppress all vanilla bookmarks
+# Total conversion — only Mistborn bookmarks should appear
+############################################
+
+bm_867_iberia = { is_playable = no }
+bm_867_northmen = { is_playable = no }
+bm_867_adventurers = { is_playable = no }
+bm_867_carolingians = { is_playable = no }
+bm_867_mandalas = { is_playable = no }
+bm_1066_hastings = { is_playable = no }
+bm_1066_rags_to_riches = { is_playable = no }
+bm_1066_iberia = { is_playable = no }
+bm_1066_laamps = { is_playable = no }
+bm_1178_call_of_the_empire = { is_playable = no }
+bm_1178_swords_of_faith = { is_playable = no }
+bm_1066_nomads = { is_playable = no }
+bm_1178_nomads = { is_playable = no }
+bm_867_china = { is_playable = no }
+bm_1066_china = { is_playable = no }
+bm_1178_china = { is_playable = no }
+bm_1066_japan = { is_playable = no }
+bm_1178_genpei = { is_playable = no }