feat(ambience): underwater shaders + audio manager + main menu + CC0 assets

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

scripts/ambience/MainMenu.gd

@@ -0,0 +1,29 @@
+extends Control
+
+@onready var _options_popup: Window = $OptionsPopup
+@onready var _music_slider: HSlider = $OptionsPopup/VBox/MusicSlider
+@onready var _sfx_slider: HSlider = $OptionsPopup/VBox/SFXSlider
+
+func _ready() -> void:
+	AudioManager.play_music("underwater_theme.mp3")
+	AudioManager.play_ambient_loop("underwater_ambient.ogg")
+
+
+func _on_dive_pressed() -> void:
+	get_tree().change_scene_to_file("res://scenes/Main.tscn")
+
+
+func _on_options_pressed() -> void:
+	_options_popup.popup_centered()
+
+
+func _on_quit_pressed() -> void:
+	get_tree().quit()
+
+
+func _on_music_slider_value_changed(value: float) -> void:
+	AudioManager.set_music_volume(linear_to_db(value))
+
+
+func _on_sfx_slider_value_changed(value: float) -> void:
+	AudioManager.set_sfx_volume(linear_to_db(value))