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/ui/main_menu.gd

@@ -0,0 +1,50 @@
+extends Control
+## #258: Main menu — New Game / Continue / Quit.
+## New Game: generates per-game save directory (D-085), starts game.
+## Continue: loads most recent save directory (#554 full loading screen deferred).
+
+const GAME_SCENE := "res://scenes/main.tscn"
+
+const BG_COLOR := Color(0.05, 0.05, 0.08, 1.0)
+const TITLE_COLOR := Color("#c8d0e0")
+const SUBTITLE_COLOR := Color("#8890a0")
+const BTN_NORMAL_COLOR := Color("#e8c547")
+const BTN_DISABLED_COLOR := Color("#4a5060")
+const FONT_SIZE_TITLE := 36
+const FONT_SIZE_SUBTITLE := 14
+const FONT_SIZE_BTN := 15
+
+@onready var _new_game_btn: Button = $VBox/NewGameBtn
+@onready var _continue_btn: Button = $VBox/ContinueBtn
+@onready var _quit_btn: Button = $VBox/QuitBtn
+
+
+func _ready() -> void:
+	_new_game_btn.pressed.connect(_on_new_game)
+	_continue_btn.pressed.connect(_on_continue)
+	_quit_btn.pressed.connect(_on_quit)
+	_refresh_continue_state()
+
+
+func _refresh_continue_state() -> void:
+	var saves := SessionManager.list_game_dirs()
+	_continue_btn.disabled = saves.is_empty()
+
+
+func _on_new_game() -> void:
+	SessionManager.new_game()
+	get_tree().change_scene_to_file(GAME_SCENE)
+
+
+func _on_continue() -> void:
+	# #554: Full loading screen blocked until server SaveCommand lands.
+	# For now: automatically load the most recent game directory.
+	var saves := SessionManager.list_game_dirs()
+	if saves.is_empty():
+		return
+	SessionManager.resume_game(saves[0].game_id)
+	get_tree().change_scene_to_file(GAME_SCENE)
+
+
+func _on_quit() -> void:
+	get_tree().quit()