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>

packaging/make_icon.py

@@ -0,0 +1,51 @@
+"""Render assets/icon.svg to assets/icon.ico (PNG-compressed entries at the usual Windows sizes).
+
+Run with: uv run python packaging/make_icon.py
+"""
+
+import struct
+import sys
+from pathlib import Path
+
+from PySide6.QtCore import QBuffer, QByteArray, QIODevice, Qt
+from PySide6.QtGui import QGuiApplication, QImage, QPainter
+from PySide6.QtSvg import QSvgRenderer
+
+SIZES = [16, 24, 32, 48, 64, 128, 256]
+ROOT = Path(__file__).resolve().parent.parent
+
+
+def render_png(renderer: QSvgRenderer, size: int) -> bytes:
+    img = QImage(size, size, QImage.Format.Format_ARGB32)
+    img.fill(Qt.GlobalColor.transparent)
+    painter = QPainter(img)
+    painter.setRenderHint(QPainter.RenderHint.Antialiasing)
+    renderer.render(painter)
+    painter.end()
+    data = QByteArray()
+    buf = QBuffer(data)
+    buf.open(QIODevice.OpenModeFlag.WriteOnly)
+    img.save(buf, "PNG")
+    return bytes(data)
+
+
+def main() -> None:
+    QGuiApplication(sys.argv[:1])
+    renderer = QSvgRenderer(str(ROOT / "assets/icon.svg"))
+    pngs = [(size, render_png(renderer, size)) for size in SIZES]
+    # ICONDIR header, then one ICONDIRENTRY per image, then the PNG data.
+    out = struct.pack("<HHH", 0, 1, len(pngs))
+    offset = 6 + 16 * len(pngs)
+    for size, png in pngs:
+        dim = 0 if size >= 256 else size  # 0 means 256
+        out += struct.pack("<BBBBHHII", dim, dim, 0, 0, 1, 32, len(png), offset)
+        offset += len(png)
+    out += b"".join(png for _, png in pngs)
+    (ROOT / "assets/icon.ico").write_bytes(out)
+    # The app loads this at runtime (window/taskbar icon), so it lives inside the package.
+    (ROOT / "src/mtga_assistant/icon.png").write_bytes(dict(pngs)[256])
+    print(f"wrote assets/icon.ico ({len(out)} bytes) and src/mtga_assistant/icon.png")
+
+
+if __name__ == "__main__":
+    main()