readability: extract magic constants, fix Go jargon

- goos/goarch -> os_name/arch_name (Go ecosystem jargon)
- BUTLER_USER_AGENT now reads version from plugin.json at runtime
- Added named constants: CONNECTION_TIMEOUT_ITERATIONS,
  CONNECTION_POLL_DELAY_MS, PERCENTAGE_SCALE, LAUNCH_WATCH_INTERVAL_SEC,
  UPDATE_CHECK_INTERVAL_SEC, BUTLERD_BOOT_MAX_ATTEMPTS,
  BUTLERD_BOOT_RETRY_DELAY_SEC, LAUNCH_WATCH_TIMEOUT_MS,
  EXECUTE_PERMISSION_BIT
- Removed BYTES_PER_MB (inline 1024 * 1024 is clearer)

core/itch_client.gd

@@ -29,6 +29,9 @@ const butler_dir := "user://butler"
 ## survive plugin updates untouched.
 const games_dir := "user://butler/games"
 const CACHE_DIR := "itch"
+const CONNECTION_TIMEOUT_ITERATIONS := 50
+const CONNECTION_POLL_DELAY_MS := 20
+const PERCENTAGE_SCALE := 100
 
 enum STATE {
 	BOOT,
@@ -101,7 +104,7 @@ func bootstrap() -> void:
 		"--keep-alive",
 		"--dbpath", dbpath,
 		"--address", "https://itch.io",
-		"--user-agent", "OpenGamepadUI-itch/0.1.0",
+		"--user-agent", _get_user_agent(),
 		"--destiny-pid", str(OS.get_process_id()),
 	]
 
@@ -138,21 +141,21 @@ func _migrate_butler(dest_dir: String) -> void:
 ## Downloads a butler binary for this platform from itch's broth distribution
 ## channel (the same one the official itch.io app uses to self-update).
 func _install_butler() -> bool:
-	var goos := "linux"
-	var goarch := "amd64"
+	var os_name := "linux"
+	var arch_name := "amd64"
 	if OS.get_name() == "Windows":
-		goos = "windows"
+		os_name = "windows"
 	if OS.get_name() == "macOS":
-		goos = "darwin"
+		os_name = "darwin"
 	# Engine.get_architecture_name() is only available on newer Godot 4.x
 	# builds; guard it so this still works if OpenGamepadUI is running on an
 	# older engine version.
 	if Engine.has_method("get_architecture_name"):
 		var arch: String = Engine.get_architecture_name()
 		if "arm64" in arch or "aarch64" in arch:
-			goarch = "arm64"
+			arch_name = "arm64"
 
-	var platform_slug := goos + "-" + goarch
+	var platform_slug := os_name + "-" + arch_name
 	var latest_url := "/".join([broth_base, platform_slug, "LATEST"])
 
 	var http := HTTPRequest.new()
@@ -204,6 +207,17 @@ func _install_butler() -> bool:
 	return true
 
 
+func _get_user_agent() -> String:
+	var version := "unknown"
+	var file := FileAccess.open("res://plugin.json", FileAccess.READ)
+	if file:
+		var json: Variant = JSON.parse_string(file.get_as_text())
+		file.close()
+		if json is Dictionary:
+			version = json.get("plugin.version", "unknown")
+	return "OpenGamepadUI-itch/" + version
+
+
 # ---------------------------------------------------------------------------
 # Wire protocol: spawn -> read handshake off stdout -> connect TCP -> auth
 # ---------------------------------------------------------------------------
@@ -266,10 +280,10 @@ func _connect_and_authenticate(address: String, secret: String) -> void:
 		return
 
 	# Wait for the connection to establish
-	var timeout := 50
+	var timeout := CONNECTION_TIMEOUT_ITERATIONS
 	while socket.get_status() == StreamPeerTCP.STATUS_CONNECTING and timeout > 0:
 		socket.poll()
-		OS.delay_msec(20)
+		OS.delay_msec(CONNECTION_POLL_DELAY_MS)
 		timeout -= 1
 	if socket.get_status() != StreamPeerTCP.STATUS_CONNECTED:
 		logger.error("Timed out connecting to butlerd")
@@ -790,8 +804,8 @@ func _install(game: Dictionary, cave_id: String, options: Dictionary) -> bool:
 		var progress: float = p.get("progress", 0.0)
 		install_progressed.emit.call_deferred(
 			cave_id if cave_id != "" else str(game_id),
-			int(progress * 100),
-			100
+			int(progress * PERCENTAGE_SCALE),
+			PERCENTAGE_SCALE
 		)
 	rpc_notification.connect(on_notification)