Add dArkOS platform support

[misantronic]

Summary

• Formalizes a community-contributed dArkOS fork of the KNULLI installer into a proper linux/darkos/ bundle (mirrors the KNULLI/Onion/muOS per-device structure)
• Shared Python source now recognizes /home/ark as an alternate platform root, so autostart and uninstall show up correctly in the menu (previously silently hidden since detection was hardcoded to /userdata/system)
• dArkOS is systemd-native (no custom.sh boot hook), so autostart installs a raofflineproxy-autostart.service unit instead; toggling autostart from the menu only flips a config flag once the unit is installed, so no root is needed at runtime
• Installer auto-installs python3-pygame via apt when run as root and pygame isn't present
• Docs updated with a dArkOS tab alongside KNULLI/Onion/muOS

Not yet verified on real dArkOS hardware by me — built and payload-checked locally, but needs on-device testing before it's considered stable.

Test plan

• python3 -m pytest linux/tests/ — new darkOS autostart/config-path cases pass; pre-existing macOS-only failures unchanged
• ./linux/darkos/build_bundle.sh builds successfully and produces a valid self-extracting installer
• Extracted the installer payload and confirmed the darkOS code branches are present in the packaged source
• On-device install/uninstall/autostart verification on dArkOS hardware