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/build_windows.ps1

@@ -0,0 +1,13 @@
+# Build the Windows app folder and a zip of it:  dist\MTGA Assistant\  and  dist\MTGA-Assistant-windows.zip
+# Requires uv (https://docs.astral.sh/uv/). Run from anywhere:
+#     powershell -ExecutionPolicy Bypass -File packaging\build_windows.ps1
+$ErrorActionPreference = "Stop"
+Set-Location (Split-Path $PSScriptRoot -Parent)
+
+uv sync --group build
+uv run pyinstaller --noconfirm --clean packaging/mtga-assistant.spec
+
+$zip = "dist/MTGA-Assistant-windows.zip"
+if (Test-Path $zip) { Remove-Item $zip }
+Compress-Archive -Path "dist/MTGA Assistant" -DestinationPath $zip
+Write-Host "Built dist\MTGA Assistant\MTGA Assistant.exe and $zip"