Add thank-you ending screen and a run timer

Defeating the bear (the run's final boss) now plays a dedicated
"Thank You For Playing" clip on the completion screen instead of the
generic bull-win video, with the run's finish time shown in place of
the usual "Bull Won!" title.

A run clock also starts when the player enters the arena (the run's
first fight), shown top-left on desktop only, and freezes the instant
the bear falls. Re-entering the arena after finishing a run restarts
the clock instead of leaving it stuck on the previous run's time.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MXBNaEBmrYDSMcv4NgUWtV

levels/to_the_arena_level.gd

@@ -8,5 +8,6 @@ func _ready() -> void:
 func _on_body_entered(body: Node3D) -> void:
 	if not body.is_in_group("player"):
 		return
+	Run.start_run_timer()
 	Run.choose_level(&"arena")
 	get_tree().change_scene_to_file(Run.active_level().scene_path)