Add a main menu scene (#47)

MainMenu.tscn is now project.godot's entry point: Play / Map Editor
(disabled, explains itself - #48 wires it up) / Quit (hidden in web
builds), styled like the rest of the game's menus and reusing the
starfield shader for its background.

Play calls GameState.reset_new_game(), so a second playthrough in the
same process starts fresh instead of inheriting whatever the previous
run left in the autoload - existing tests still load Main.tscn
directly and are unaffected.

Part of the #40 conquest-mechanic breakdown.

MainMenu.gd

@@ -0,0 +1,33 @@
+extends Node2D
+
+@onready var play_button: IconButton = $UI/PlayButton
+@onready var editor_button: IconButton = $UI/EditorButton
+@onready var quit_button: IconButton = $UI/QuitButton
+@onready var status_label: Label = $UI/StatusLabel
+
+
+func _ready() -> void:
+	play_button.pressed.connect(_on_play_pressed)
+	editor_button.pressed.connect(_on_editor_pressed)
+	# #48 wires the real map editor scene up to this button; until then it
+	# stays visible but explains itself instead of silently doing nothing
+	# or pointing at a scene that doesn't exist yet.
+	editor_button.enabled = false
+	quit_button.pressed.connect(_on_quit_pressed)
+	quit_button.visible = not OS.has_feature("web")
+
+
+func _on_play_pressed() -> void:
+	GameState.reset_new_game()
+	get_tree().change_scene_to_file("res://Main.tscn")
+
+
+func _on_editor_pressed() -> void:
+	if editor_button.enabled:
+		get_tree().change_scene_to_file("res://PlanetSetupEditor.tscn")
+		return
+	status_label.text = "Map editor coming soon"
+
+
+func _on_quit_pressed() -> void:
+	get_tree().quit()