Added gameplay_input manager.

Refactored menus, player, and escape_capsule to utilize new system.
Actions defined in Godot project file with default bindings.
Player/Vehicles now react to actions and GameplayInput states instead of polling Input directly.
They respond to commands via `_handle_gameplay_command` and should not implement _unhandled_input directly.
Kept some debugging input handling code in `player.gd` for things like F3/F5/F4/PageUp/PageDown.
We may want to move much of this out of `player.gd` into something that handles the gamestate more generally.

Scenes/Scripts/main_menu.gd

@@ -7,6 +7,7 @@ extends Control
 
 func _ready() -> void:
 	Input.mouse_mode = Input.MOUSE_MODE_VISIBLE
+	GameplayInput.set_menu_open(self, true)
 	if new_game_button:
 		new_game_button.pressed.connect(_on_new_game)
 	if continue_button:
@@ -15,6 +16,12 @@ func _ready() -> void:
 		quit_button.pressed.connect(_on_quit)
 	if continue_button:
 		continue_button.disabled = not GameState.has_save
+	new_game_button.grab_focus()
+
+
+## Removes menu ownership when changing scenes so gameplay can resume.
+func _exit_tree() -> void:
+	GameplayInput.set_menu_open(self, false)
 
 
 func _on_new_game() -> void: