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/ui/main_window.py

@@ -24,6 +24,7 @@ from PySide6.QtWidgets import (
     QWidget,
 )
 
+from .. import paths
 from ..carddb import CardDB
 from ..gamestate import MatchTracker
 from ..history import HistoryRecorder, HistoryStore, import_log, replay_game
@@ -405,7 +406,7 @@ class MainWindow(QMainWindow):
         messages = []
         if not self.cards.available:
             messages.append("MTGA card database not found: card names are unavailable. "
-                            "Set mtga_dir in ~/.config/mtga-assistant/config.toml.")
+                            f"Set mtga_dir in {paths.CONFIG_FILE}.")
         # MTGA sometimes leaves oversized messages out of the log. Usually later messages fill the
         # gap in; only flag it (quietly) while some of my cards are still not accounted for.
         unaccounted = 0