Make the app cross-platform and add a Windows build

- paths: per-platform detection behind an injectable Env. Windows finds
  Player.log in the user profile and the install via Steam (registry),
  the standalone installer, Epic, or the uninstall registry. App dirs
  come from platformdirs (unchanged XDG locations on Linux).
- logtail: open Player.log only while reading, so Windows can still
  rotate it when MTGA restarts.
- carddb: on Windows read a private copy of the card database so MTGA's
  updater can replace the original.
- App icon (SVG source + generated ICO/PNG), Windows taskbar identity,
  and a Windows-safe timer glyph.
- PyInstaller spec, launcher and PowerShell build script producing a
  one-folder windowed build and zip.
- Tests for path detection, the log follower and the card-DB copy;
  README covers Windows usage, building and per-OS locations.

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

src/mtga_assistant/app.py

@@ -6,10 +6,11 @@ import argparse
 import sys
 from pathlib import Path
 
+from PySide6.QtGui import QIcon
 from PySide6.QtWidgets import QApplication
 
 from . import paths
-from .carddb import CardDB
+from .carddb import open_card_db
 from .history import HistoryStore
 from .images import ImageCache
 from .ui import theme
@@ -30,14 +31,20 @@ def main() -> int:
     if args.mtga_dir:
         cfg.mtga_dir = args.mtga_dir
 
+    if sys.platform == "win32":
+        # Own taskbar identity (icon and grouping) instead of python.exe's.
+        import ctypes
+
+        ctypes.windll.shell32.SetCurrentProcessExplicitAppUserModelID("mtga-assistant")
     app = QApplication([sys.argv[0], *qt_args])
+    app.setWindowIcon(QIcon(str(Path(__file__).with_name("icon.png"))))
     app.setApplicationName("MTGA Assistant")
     app.setDesktopFileName("mtga-assistant")  # Wayland app_id, usable in Hyprland window rules
     theme.apply(app)
 
     mtga_dir = paths.find_mtga_dir(cfg)
     db_path = paths.find_card_db(mtga_dir) if mtga_dir else None
-    cards = CardDB(db_path)
+    cards = open_card_db(db_path, paths.CACHE_DIR / "carddb")
     images = ImageCache(paths.CACHE_DIR, cards, cfg.cache_max_mb * 1024 * 1024)
 
     history = HistoryStore(paths.DATA_DIR / "history.db", cfg.history_max_mb)