refactor(m4c): rename GameFlow coordinator to Main, move to scenes/Main.tscn

The entry-point coordinator read as an internal name ("GameFlow" buried in
scenes/flow/), so it wasn't obvious THIS is the scene that boots first — the
title screen looked like the entry point. Renamed to the Godot-conventional
Main (scenes/Main.tscn + scripts/main.gd) so the boot scene is self-evident.
TitleScreen is unchanged and still shown BY Main as its first screen.

Also drops the hand-written gd_scene uid (now matches sibling scenes) —
closes the Minor review finding. Full suite 327/327 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/scenes/Main.tscn

@@ -0,0 +1,6 @@
+[gd_scene load_steps=2 format=3]
+
+[ext_resource type="Script" path="res://scripts/main.gd" id="1"]
+
+[node name="Main" type="Node"]
+script = ExtResource("1")