Rewrite README with logo and screenshots

Add the logo and screenshots, restructure into features, installation,
usage, configuration and development, and bring everything up to date
(background mode, installers, menus, limitations). The project
description now covers Linux and Windows.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pyproject.toml

@@ -1,7 +1,7 @@
 [project]
 name = "mtga-assistant"
 version = "0.1.0"
-description = "Native Linux deck and game tracker for MTG Arena, driven by Player.log"
+description = "Deck and game tracker for MTG Arena on Linux and Windows, driven by Player.log"
 readme = "README.md"
 requires-python = ">=3.12"
 dependencies = [