Keep the screensaver off while Firefox-family browsers play media

[michielvandermeer]

Fixes #7220. Fixes #7199. Related to #6475.

What happens today

Quattro's idle service uses IdleMonitor { respectInhibitors: true }, which only honors Wayland zwp_idle_inhibit_manager_v1 inhibitors. Firefox, Zen, and LibreWolf request idle inhibit over the GTK portal / org.freedesktop.ScreenSaver while a <video> is playing. Nothing owns that bus name since hypridle was replaced, so the call is a silent no-op and the screensaver comes up over YouTube, Plex, and the rest — windowed or fullscreen.

Chromium-based browsers already speak the Wayland protocol directly and are unaffected.

Reproduced on Omarchy 4.0.1 with Firefox 154 playing Plex in a windowed window: hyprctl clients showed inhibitingIdle: false while MPRIS PlaybackStatus was Playing. Tagging the window noidle (the existing idle_inhibit = always rule) immediately set inhibitingIdle: true.

What this does

1. The idle service watches MPRIS. While a Firefox-family player reports Playing, it holds idle off the same way Stay Awake does — including cancelling an in-flight screensaver cycle. Pause/stop releases after 2s so a blip does not start the timer. Spotify, Chromium, and other players are left alone.
2. Firefox-based browser windows also get idle_inhibit = fullscreen, so fullscreen video that never publishes MPRIS still holds idle off.
3. idle toggle now flips stayAwake rather than the derived idleEnabled flag, so Stay Awake still toggles correctly while media is inhibiting.

Idle browsing with Firefox open is unchanged. You only skip the screensaver while media is actually playing.

Why not the other open PRs

• Force Firefox's media wakelock onto the Wayland idle-inhibit protocol #7877 pins MOZ_WAKE_LOCK_TYPE=WaylandIdleInhibit. That is the right native backend when it works, but Firefox's Wayland inhibit is attached to the focused surface and can fail (or stick failed) if playback starts unfocused. This PR does not need Firefox to inhibit anything.
• Honor D-Bus idle inhibits again: screensaver no longer fires over video playback #8452 / Re-implement D-Bus idle-inhibit support dropped in the Quattro rewrite #6572 restore org.freedesktop.ScreenSaver for every D-Bus client (VLC, Stremio, …). That is the spec-complete fix for [Quattro] D-Bus idle inhibits are silently dropped since hypridle was replaced — screensaver fires during video playback (reproduced with Zen browser) #6475 and is complementary. This PR is the smaller, already-running-in-shell path that covers the reported Firefox/Zen cases without a new daemon.

Test

• bash test/shell.d/idle-test.sh — player matching, idleEnabled gating, and QML wiring.
• ./test/shell — idle file green. The same three omarchy-pkgs-checkout failures as on quattro (config-test.sh, snapper-test.sh, unowned-system-paths-test.sh); launch-about-test.sh, runtime-smoke-test.sh, and theme-install-guards-test.sh also fail here on unmodified quattro (no compositor animation / live IPC / network).

Live check on this machine: with Plex playing in windowed Firefox, idle status reports mediaInhibiting: true and the screensaver no longer starts.