feat: add post-processing pass and video settings menu

Enable glow/bloom, color adjustments, and MSAA+FXAA across all three
arenas so emissive ship/station accents actually bleed light and edges
read cleanly. Expose the player-facing knobs (anti-aliasing mode, glow
intensity, brightness) through a new Settings screen off the main menu,
persisted via a VideoSettings autoload that scales each arena's own
tuned Environment values on load rather than overwriting them.

Game/scripts/video_settings.gd

@@ -0,0 +1,65 @@
+extends Node
+
+# Autoload: persisted player-facing video preferences (AA, glow, brightness).
+# AA is a Viewport-wide setting applied immediately via apply_aa(). Glow and
+# brightness instead scale each arena's own tuned Environment values (see
+# arena.gd's _ready(), which calls apply_to_environment() once per arena
+# load) rather than overwriting them outright, so the per-arena bloom tuning
+# in arena_01/02/03.tscn survives underneath the user's preference.
+
+enum AAMode { OFF, FXAA, MSAA, MSAA_FXAA }
+
+const SETTINGS_PATH := "user://settings.cfg"
+
+var aa_mode: AAMode = AAMode.MSAA_FXAA
+var glow_scale: float = 1.0
+var brightness: float = 1.0
+
+
+func _ready() -> void:
+	_load()
+	apply_aa()
+
+
+func _load() -> void:
+	var cfg := ConfigFile.new()
+	if cfg.load(SETTINGS_PATH) != OK:
+		return
+	aa_mode = cfg.get_value("video", "aa_mode", aa_mode) as AAMode
+	glow_scale = cfg.get_value("video", "glow_scale", glow_scale)
+	brightness = cfg.get_value("video", "brightness", brightness)
+
+
+func save() -> void:
+	var cfg := ConfigFile.new()
+	cfg.set_value("video", "aa_mode", aa_mode)
+	cfg.set_value("video", "glow_scale", glow_scale)
+	cfg.set_value("video", "brightness", brightness)
+	cfg.save(SETTINGS_PATH)
+
+
+func apply_aa() -> void:
+	var viewport := get_tree().root
+	match aa_mode:
+		AAMode.OFF:
+			viewport.msaa_3d = Viewport.MSAA_DISABLED
+			viewport.screen_space_aa = Viewport.SCREEN_SPACE_AA_DISABLED
+		AAMode.FXAA:
+			viewport.msaa_3d = Viewport.MSAA_DISABLED
+			viewport.screen_space_aa = Viewport.SCREEN_SPACE_AA_FXAA
+		AAMode.MSAA:
+			viewport.msaa_3d = Viewport.MSAA_4X
+			viewport.screen_space_aa = Viewport.SCREEN_SPACE_AA_DISABLED
+		AAMode.MSAA_FXAA:
+			viewport.msaa_3d = Viewport.MSAA_4X
+			viewport.screen_space_aa = Viewport.SCREEN_SPACE_AA_FXAA
+
+
+# Called once by each arena's _ready() to fold the user's glow/brightness
+# preference into that arena's own baked Environment tuning.
+func apply_to_environment(env: Environment) -> void:
+	if env == null:
+		return
+	env.glow_enabled = glow_scale > 0.0
+	env.glow_intensity *= glow_scale
+	env.adjustment_brightness *= brightness