Fix the installer; start the desktop without the Steam client's environment

install.sh: a comment with an apostrophe inside a single-quoted command
(from the distrobox pin) ended the quote and broke the script at step 1.

The VR launcher started the desktop with the Steam client's environment,
including LD_LIBRARY_PATH to Steam's runtime, whose libavcodec has no
H.264 decoder: VLC in the desktop couldn't play most videos. The session
script now drops the client's variables before starting anything, so apps
use the system's libraries and codecs.

The README and the installer now recommend setting Steam's Sleep after
(When Plugged In and Idle) to Never; Steam suspends the Frame after an
hour otherwise, even while charging.

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

docs/design.md

@@ -100,6 +100,10 @@ Keyboards aren't grabbed by default, because a grabbed keyboard's keys went into
 
 The session is modeled on SteamOS's `steamos-nested-desktop` and runs beside it. It has its own runtime directory, config (`~/.config/frametop`), and state, so it never disturbs the stock desktop's layout or panels. It runs on a private D-Bus from `dbus-run-session`, which has two consequences. KDE only launches apps in systemd scopes when systemd is on the session bus, so everything started in the desktop lands in its systemd unit, and stopping the unit would kill all of it; `session/keep-apps.sh` moves those programs out first. And tools that need the real user bus, like podman and `distrobox-host-exec`, have to be pointed at it explicitly.
 
+The VR launcher starts the session from the Steam client, and the client's environment came along: `LD_LIBRARY_PATH` pointing at Steam's own runtime, whose `libavcodec` has no H.264 decoder, so VLC in the desktop couldn't play most videos, plus the client's overlay and launch settings. The session script drops the client's variables before it starts anything. SteamOS's global Mesa settings (`/usr/share/deckard/mesavars.sh`) stay, and the gamescope session's Vulkan layer (`ENABLE_GAMESCOPE_WSI`) is only kept for the gamescope backend.
+
+Steam, not systemd, suspends the Frame: after `system_idle_suspend_ac_sec` (an hour by default) without input on AC power, it logs `Switching to power state: k_ESystemPowerState_Sleep` and suspends, even while charging. It's a Steam setting (Settings → Power → When Plugged In and Idle → Sleep after), so the README recommends setting it to Never. SteamVR's standby, which turns the displays off when the headset comes off, is separate.
+
 Flatpak apps need `XDG_DATA_DIRS` to include Flatpak's exports, or Plasma opens Discover instead of launching them, so the session sources `/etc/profile.d/flatpak.sh`.
 
 A podman container's monitor process (conmon) stays in the cgroup of whatever started the container, and `distrobox enter` starts it on demand. When a Frametop service happened to start the `dev` container, stopping that service stopped the container and everything in it, including the desktop's compositor. `scripts/container-up.sh` starts the container in a systemd scope of its own before anything enters it.