feat: 实现存档界面与场景切换，优化文字渲染

- 新增 SavePanel/SaveSlot 存档选择界面，支持已有存档与新建存档
- 新增 GameState 单例与 MainGame 入口，按参数区分新游戏/读档
- 主菜单接入中文字体，返回/进入存档面板流程打通
- 开启 2D/3D MSAA、TAA 与屏幕空间 AA，消除放大文字锯齿
- 设置 1920x1080 视口与中文本地化过滤

assets/fonts/dingliezhuhaifont-20240831GengXinBan)-2.ttf.import

@@ -0,0 +1,36 @@
+[remap]
+
+importer="font_data_dynamic"
+type="FontFile"
+uid="uid://drt448qncx27p"
+path="res://.godot/imported/dingliezhuhaifont-20240831GengXinBan)-2.ttf-7b6c32418ce639305b17af2723dddc58.fontdata"
+
+[deps]
+
+source_file="res://assets/fonts/dingliezhuhaifont-20240831GengXinBan)-2.ttf"
+dest_files=["res://.godot/imported/dingliezhuhaifont-20240831GengXinBan)-2.ttf-7b6c32418ce639305b17af2723dddc58.fontdata"]
+
+[params]
+
+Rendering=null
+antialiasing=1
+generate_mipmaps=false
+disable_embedded_bitmaps=true
+multichannel_signed_distance_field=false
+msdf_pixel_range=8
+msdf_size=48
+allow_system_fallback=true
+force_autohinter=false
+modulate_color_glyphs=false
+hinting=3
+subpixel_positioning=4
+keep_rounding_remainders=true
+oversampling=0.0
+Fallbacks=null
+fallbacks=[]
+Compress=null
+compress=true
+preload=[]
+language_support={}
+script_support={}
+opentype_features={}