Harden wallpaper reads against special-file DoS

[zyads]

UserData::load_wallpapers_as_user reads each wallpaper with a plain fs::read on a path taken from that user's own cosmic-bg config, and the greeter daemon loads every user's config at the login/lock screen. Pointing that path at a FIFO blocks the read forever, and a device like /dev/zero reads without bound until the daemon is OOM-killed, so a single unprivileged user can deny the graphical login/lock screen to the whole machine.

Read wallpapers through a helper that opens with O_NONBLOCK, requires a regular file, and caps the read at 256 MiB, mirroring the defensive handling already applied to the AccountsService icon read. Verified with cargo check -p cosmic-greeter-daemon.

🤖 Generated with Claude Code

---

• I have disclosed use of any AI generated code in my commit messages.
  • If you are using an LLM, and do not fully understand the changes it is making to the code base, do not create a PR.
  • In our experience, AI generated code often results in overly complex code that lacks enough context for a proper fix or feature inclusion. This results in considerably longer code reviews. Due to this, AI authored or partially authored PRs may be closed without comment.
• I understand these changes in full and will be able to respond to review comments.
• My change is accurately described in the commit message.
• My contribution is tested and working as described.
• I have read the Developer Certificate of Origin and certify my contribution under its conditions.