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/settings_menu.gd

@@ -0,0 +1,70 @@
+extends Control
+
+# Settings screen: a small set of player-facing video knobs on top of
+# VideoSettings (the autoload holding + persisting them). Anti-aliasing
+# applies immediately since it's a Viewport-wide setting; glow/brightness
+# apply the next time an arena loads (see arena.gd), since they scale each
+# arena's own tuned Environment values rather than something viewport-wide.
+
+const MAIN_MENU_SCENE_PATH := "res://scenes/main_menu.tscn"
+
+const AA_OPTIONS := [
+	{"name": "Off", "mode": VideoSettings.AAMode.OFF},
+	{"name": "FXAA", "mode": VideoSettings.AAMode.FXAA},
+	{"name": "MSAA 4x", "mode": VideoSettings.AAMode.MSAA},
+	{"name": "MSAA 4x + FXAA", "mode": VideoSettings.AAMode.MSAA_FXAA},
+]
+
+@onready var aa_dropdown: OptionButton = %AADropdown
+@onready var glow_slider: HSlider = %GlowSlider
+@onready var glow_value_label: Label = %GlowValueLabel
+@onready var brightness_slider: HSlider = %BrightnessSlider
+@onready var brightness_value_label: Label = %BrightnessValueLabel
+
+
+func _ready() -> void:
+	aa_dropdown.clear()
+	var selected := 0
+	for i in AA_OPTIONS.size():
+		aa_dropdown.add_item(AA_OPTIONS[i]["name"])
+		if AA_OPTIONS[i]["mode"] == VideoSettings.aa_mode:
+			selected = i
+	aa_dropdown.select(selected)
+
+	glow_slider.value = VideoSettings.glow_scale
+	brightness_slider.value = VideoSettings.brightness
+	_update_glow_label()
+	_update_brightness_label()
+
+
+func _update_glow_label() -> void:
+	glow_value_label.text = "%d%%" % roundi(glow_slider.value * 100.0)
+
+
+func _update_brightness_label() -> void:
+	brightness_value_label.text = "%d%%" % roundi(brightness_slider.value * 100.0)
+
+
+func _on_aa_dropdown_item_selected(index: int) -> void:
+	VideoSettings.aa_mode = AA_OPTIONS[index]["mode"]
+	VideoSettings.apply_aa()
+
+
+func _on_glow_slider_value_changed(value: float) -> void:
+	VideoSettings.glow_scale = value
+	_update_glow_label()
+
+
+func _on_brightness_slider_value_changed(value: float) -> void:
+	VideoSettings.brightness = value
+	_update_brightness_label()
+
+
+func _on_back_pressed() -> void:
+	VideoSettings.save()
+	get_tree().change_scene_to_file(MAIN_MENU_SCENE_PATH)
+
+
+func _unhandled_input(event: InputEvent) -> void:
+	if event.is_action_pressed("ui_cancel"):
+		_on_back_pressed()