fix: six bugs found testing main on a real Pi (#641)

* fix: six bugs found testing main on a real Pi (ledpi)

- Stopping the service now runs cleanup. systemd stops ledmatrix.service
  with SIGTERM, whose default action ended Python before run()'s finally
  block, so the update worker, Vegas and the panel were never torn down.
  main() now turns SIGTERM into KeyboardInterrupt, the Ctrl-C path.
- "Now showing" no longer turns into "unknown". display_current_state was
  only written on a mode change and the web UI reads it with max_age=120,
  so a live game or a single plugin on screen for longer read as unknown.
  It is republished every 30 s while unchanged.
- Switching Vegas on in the web UI works when it was off at startup. The
  coordinator was only created at startup; the config watcher now flags it
  and the render thread creates it.
- configure_web_sudo.sh finds reboot and poweroff in /usr/sbin. Run as the
  web user it could not, silently dropped their rules and still said it
  granted them, so the web UI's Reboot/Shutdown stopped working.
- check_system_compatibility.sh reports installed packages as installed.
  `dpkg -l | grep -q` under pipefail failed when grep exited early.
- A network failure fetching GitHub repo info logs a WARNING, not ERROR.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* docs(changelog): fixes found testing on a Pi

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* test: run the Linux-only script tests correctly

The sbin-lookup test set PATH=/nonexistent and then could not find bash
itself; call it by absolute path. The dpkg-query stub read $4, but the
package name is the third (last) argument. Both now pass on a Pi.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(display): cover the follower, long-render and startup cases

Review follow-ups on the ledpi fixes:
- The pending Vegas start is applied before the sync-follower branch too
  (_apply_pending_vegas_init), which skips _is_vegas_mode_active() while a
  follower is connected but needs the coordinator for the leader's image.
- _service_pending_changes(), which runs inside Vegas iterations and long
  screens, republishes a stale display_current_state as well; the main
  loop alone could be away for a 240 s Vegas iteration.
- The SIGTERM handler is installed after DisplayController() is built, so a
  stop during parallel plugin loading keeps the default immediate exit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

scripts/check_system_compatibility.sh

@@ -155,7 +155,10 @@ ESSENTIAL_PACKAGES=(
 
 for pkg_info in "${ESSENTIAL_PACKAGES[@]}"; do
     IFS=':' read -r pkg desc <<< "$pkg_info"
-    if dpkg -l | grep -q "^ii  $pkg "; then
+    # dpkg-query rather than `dpkg -l | grep -q`: under pipefail, grep -q
+    # exiting on its first match kills dpkg with SIGPIPE and fails the pipeline,
+    # which reported installed packages as missing.
+    if [ "$(dpkg-query -W -f='${Status}' "$pkg" 2>/dev/null)" = "install ok installed" ]; then
         print_success "$desc ($pkg) is installed"
     else
         print_warning "$desc ($pkg) not installed - will be installed during setup"