Add Linux and Windows installers

- install.sh: checks Python 3.12+, installs the app into its own
  environment, checks Qt's system libraries, installs icons and a
  --background .desktop launcher, offers start at login; --uninstall.
- install.bat / install_windows.ps1: checks Python (offering winget),
  builds with PyInstaller, installs to LOCALAPPDATA with Start Menu and
  Desktop shortcuts, offers start at login; -Uninstall.
- .gitattributes keeps .bat/.ps1 as CRLF and .sh as LF.

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

.gitattributes

@@ -0,0 +1,6 @@
+# Windows scripts need CRLF line endings.
+*.bat text eol=crlf
+*.ps1 text eol=crlf
+*.ico binary
+*.png binary
+*.sh text eol=lf