fix(media-stack panel): background only the installer in the start command

[K4r3s-git]

Bug: pmssMediaStackPanelStartCommandBuild() builds

cd HOME && rm -f -- PIDFILE && nohup /bin/bash install-media-stack.sh >/dev/null 2>&1 & echo $! > PIDFILE

The trailing & backgrounds the whole && list, not just the installer:

1. Pid file disappears. rm -f PIDFILE runs inside the background subshell, usually after the foreground echo $! > PIDFILE, so .install-media-stack-web.pid is gone. The panel never shows the install as running and falls straight to installed once the first app dirs appear.
2. The Start request blocks for the whole install. The background subshell still holds shell_exec's stdout pipe, so the POST waits for the installer to exit. On a successful install the installer restarts lighttpd/php-cgi at the end, and the request dies with HTTP 502.

This hasn't been visible because the current installer aborts early at #858 (the POST then returns after ~30 s). With a completing installer (#959 applied) the Start button returns 502.

Reproduced on a Pulsed Media seedbox with the same shell and sleep 20 as the installer:

old: returned after 20s, pidfile=MISSING
new: returned after 0s,  pidfile=44082

The Start button also returned 502 there after a complete panel-driven install.

Fix: background only the installer, keep the pid write with it, and detach stdin:

cd HOME && rm -f -- PIDFILE && { nohup /bin/bash install-media-stack.sh </dev/null >/dev/null 2>&1 & echo $! > PIDFILE; }

Test: new testStartCommandReturnsAtOnceAndKeepsPidFile runs the real command against a fake installer that sleeps 3 s. The launch must return in < 2 s and the pid file must name a live process. It fails on the old command (3.01s) and passes with the fix. mediaStackPanelTest.php: 25 tests, 0 failures (1 skipped, as before). php -l OK.

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