feat: add full Linux desktop support

[TrapEmAll]

Summary

This adds first-class Linux support across runtime behavior, desktop integration, packaging, CI, releases, tests, and documentation.

Runtime and desktop

• use XDG-native data, configuration, cache, and state directories on Linux
• preserve existing Windows paths and cross-platform portable mode
• use Linux-appropriate fonts and support both xdg-open and gio open
• provide useful startup errors when Tkinter or a graphical session is unavailable
• assign a stable UnityScraper desktop class for taskbar grouping
• move knowledge-source snapshots into the platform cache directory
• remove another custom-database test/profile side effect from BackupRepository

Linux distribution

• add setup.sh and run-unityscraper.sh for source installations
• add a cross-platform PyInstaller spec and build_linux.sh
• produce UnityScraper-Linux-x86_64.tar.gz with SHA-256 verification
• include a no-root user installer, desktop entry, 1024px icon, AppStream metadata, command symlink, and safe uninstaller
• preserve user databases, configuration, downloads, caches, and logs during uninstall

Automation

• add an Ubuntu 22.04 Linux executable job after the Windows/Linux Python matrix
• validate shell syntax, FreeDesktop metadata, and AppStream metadata
• smoke test the packaged GUI under Xvfb
• publish Windows and Linux artifacts together from version tags
• keep AppStream, Python, VERSION, and project versions synchronized

Documentation

• add LINUX.md with supported distributions, release/source installation, XDG locations, mounted-device behavior, uninstallation, and troubleshooting
• update README, architecture, contributor guidance, project status, documentation index, and changelog

Compatibility and migration

• Windows remains unchanged under %LOCALAPPDATA%\UnityScraper.
• Linux now uses the XDG Base Directory specification instead of the historical ~/.unityscraper fallback.
• Portable mode still uses UnityScraperData beside the executable when portable.mode or UNITYSCRAPER_PORTABLE=1 is present.
• The Linux release targets x86_64 glibc-based desktops. ARM64 and musl packages remain future work.
• Linux devices must already be mounted. UnityScraper does not install filesystem drivers or request elevated privileges.
• XboxUnity remains explicitly HTTP-only.

Validation

• Ruff: passed
• compileall: passed
• core tests: 60 passed
• integration tests: 31 passed
• version consistency including AppStream metadata: passed
• Markdown links: passed
• Linux shell syntax: passed
• Windows PyInstaller build and checksum: passed
• native Ubuntu PyInstaller build, desktop metadata validation, and Xvfb smoke test: performed by this PR's CI before merge