feat: Boot into gamescope or the desktop

- Menu row "Boot into: [gamescope] / desktop" under the conversion; gamescope is the default.
- Desktop: steamify-boot-desktop.service sets the autologin session to Plasma at every boot, before the login manager; switching back and forth keeps working.
- VERSION 0.10.0.

lib/boot-session.sh

@@ -0,0 +1,62 @@
+#!/bin/bash
+# "Boot into: gamescope / desktop" menu item, on top of the SteamOS
+# conversion. Gamescope is the default (the conversion's autologin, and
+# CachyOS's cachyos-gamescope-autologin resetting to it after a desktop
+# session). Desktop: a unit that sets the autologin session to Plasma at
+# every boot, before the login manager starts. Switching between the two in
+# a running system (Return to Gaming Mode, Steam's Switch to Desktop) works
+# the same either way.
+# Sourced by steamify.sh; not meant to be run on its own.
+
+BOOT_UNIT_NAME="steamify-boot-desktop.service"
+BOOT_UNIT="/etc/systemd/system/$BOOT_UNIT_NAME"
+SESSION_SYNC="/usr/local/bin/sync-steamos-session.sh"
+
+boot_status() {
+    # On = boots into the desktop.
+    [[ -f "$BOOT_UNIT" ]] && systemctl is-enabled -q "$BOOT_UNIT_NAME" 2>/dev/null
+}
+
+boot_enable() {
+    info "Setting the PC to boot into the desktop..."
+    # The sync bridge (plasma-login-manager only; not there with SDDM) copies
+    # the session into the base config; run it right away rather than rely
+    # on its path unit, which could still be pending when the login manager
+    # starts.
+    sudo tee "$BOOT_UNIT" > /dev/null << EOF
+[Unit]
+Description=Steamify CachyOS: boot into the desktop instead of gaming mode
+Before=display-manager.service sddm.service plasmalogin.service
+
+[Service]
+Type=oneshot
+ExecStart=/usr/lib/steamos/steam-set-session plasma.desktop
+ExecStart=/bin/sh -c '[ -x $SESSION_SYNC ] && exec $SESSION_SYNC || true'
+
+[Install]
+WantedBy=graphical.target
+EOF
+    sudo systemctl daemon-reload
+    sudo systemctl enable "$BOOT_UNIT_NAME" || { err "Enabling $BOOT_UNIT_NAME failed."; return 1; }
+    ok "Boots into the desktop from the next boot on (Return to Gaming Mode still works)."
+}
+
+boot_disable() {
+    info "Setting the PC to boot into gaming mode again..."
+    sudo systemctl disable "$BOOT_UNIT_NAME" 2>/dev/null
+    sudo rm -f "$BOOT_UNIT"
+    sudo systemctl daemon-reload
+    # Next boot into gamescope, as the conversion sets it up.
+    sudo /usr/lib/steamos/steam-set-session gamescope-session.desktop
+    [[ -x "$SESSION_SYNC" ]] && sudo "$SESSION_SYNC"
+    ok "Boots into gaming mode from the next boot on."
+}
+
+boot_choice() {
+    # The menu row: the chosen option in brackets.
+    if [[ "$1" == 1 ]]; then
+        echo "Boot into:  gamescope  [desktop]"
+    else
+        echo "Boot into: [gamescope]  desktop"
+    fi
+}