fix(wallpaper): bundle host exe in installer + guard against missing host

A freshly installed release could not enter Desktop Wallpaper mode — the
dashboard vanished. The installer bundled rigstats.exe and rigstats-sensor.exe
but never rigstats-wallpaper.exe, so the main app's spawn of its sibling host
failed with 'The system cannot find the file specified. (os error 2)' and the
main window stayed parked off-screen. The host binary is built by
cargo build --release but installer.nsi never copied it (debug/cargo-run work
because both exes sit in target/debug, which is why it was missed).

- installer.nsi: bundle rigstats-wallpaper.exe and delete it on uninstall.
- Defensive guard: if the host binary is missing when entering wallpaper mode,
  revert to the normal layer, restore the main window on-screen (startup may
  have already parked it), and log a clear error — so a future packaging
  regression degrades to a visible dashboard instead of a blank screen.
- Extract restore_main_window() shared by the leave-mode and guard paths.

Closes #136

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>