feat(client): game session management (#258, D-085)

Per-game save directories under user://saves/<timestamp>-<seed>/.
SessionManager autoload handles new_game(), resume_game(), quit flow.
Main menu scene with New Game / Continue / Quit buttons. Game-id
passed to server subprocess via --game-id flag.

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

client/data/ui-strings.yaml

@@ -170,6 +170,8 @@ dialogue:
 menu:
   pause_title: "Paused"
   resume: "Resume"
+  new_game: "New Game"
+  continue: "Continue"
   settings: "Settings"
   save_game: "Save"
   load_game: "Load"