Notify users when linux-wallpaperengine exits immediately

[louzt]

Summary

Split from #279 to keep the first user-visible fix small and easy to review.

This changes the linux-wallpaperengine launch path from a shell-backgrounded command to an argv Popen() process that Waypaper can observe briefly after launch. If the renderer exits immediately, Waypaper now prints a clear message and sends a desktop notification when notify-send is available.

Why

Silent linux-wallpaperengine launch exits are hard to diagnose because the previous shell-backgrounded launch path discarded stdout/stderr and did not keep a process handle that Waypaper could check.

This PR only surfaces the immediate failure to the user. It does not include the larger logging/refactor work from #279.

What changed

• Launch linux-wallpaperengine with argv Popen() instead of shell=True plus &.
• Start the renderer in a new session so Waypaper does not need shell backgrounding.
• Poll the process after a short initial delay.
• Notify the user if the renderer exited immediately.
• Add unit coverage for the running-process and immediate-exit paths.

Validation

python3 -m py_compile waypaper/*.py tests/test_linux_wallpaperengine_notification.py
python3 -m unittest discover -s tests
git diff --check

Result: 13 tests pass.

Follow-ups

The broader #279 branch still contains logging, fallback, process cleanup, and module-boundary hardening. Those can be rebased/split separately after this smaller piece is reviewed.