feat(screenshot): add screenshot viewer scene

Add SCREENSHOT scene and UI to browse screenshots from the main menu,
with localization for English and Chinese. Also allow image loading
by full path and replace scene-operation asserts with error logs.

assets/lang/en_US.json

@@ -4,6 +4,7 @@
     "menu.main.join_game": "Join Game",
     "menu.main.credits": "Credits",
     "menu.main.quit": "Quit",
+    "menu.screenshot": "Screenshots",
     "menu.main.player_name": "Player: {name}",
     "menu.main.cubed_version": "Cubed: {version}",
     "menu.pause.pause_menu": "Pause Menu",
@@ -46,5 +47,6 @@
     "item.snowy_grass_block.name": "snowy grass block",
     "item.stone.name": "stone",
     "item.water.name": "water",
-    "item.pig_spawn_egg.name": "pig spawn egg"
+    "item.pig_spawn_egg.name": "pig spawn egg",
+    "screenshot.no_screenshot": "No Screenshot"
 }

assets/lang/zh_CN.json

@@ -4,6 +4,7 @@
     "menu.main.join_game": "加入游戏",
     "menu.main.credits": "制作人员",
     "menu.main.quit": "退出",
+    "menu.screenshot": "屏幕截图",
     "menu.main.player_name": "玩家：{name}",
     "menu.main.cubed_version": "Cubed：{version}",
     "menu.pause.pause_menu": "暂停菜单",
@@ -46,5 +47,6 @@
     "item.snowy_grass_block.name": "覆雪草方块",
     "item.stone.name": "石头",
     "item.water.name": "水",
-    "item.pig_spawn_egg.name": "猪猪刷怪蛋"
+    "item.pig_spawn_egg.name": "猪猪刷怪蛋",
+    "screenshot.no_screenshot": "没有截图"
 }