fix: Fix setup bugs, add SteamOS desktop look and LED driver, split into modules

- Fix session switching: sync bridge only edits [Autologin], sync service
  no longer hits systemd's start limit, re-runs don't duplicate settings
- Make the Return to Gaming Mode shortcut work without a password prompt
- Steam desktop autostart only runs in Plasma, not inside gamescope
- Vapor theme: fix install path, wallpaper, icon and color scheme; take
  over Valve's SteamOS defaults, GTK theme, panel layout and dark mode;
  always fetch the newest steamdeck-kde-presets with checksum verification
- Steam Machine: install kernel headers and build/load the leds-valve
  driver reliably; ask before installing OpenRGB
- Split the script into lib/ modules per responsibility
- Rewrite README for end users with a technical details section; add
  AGENTS.md

lib/steam-desktop.sh

@@ -0,0 +1,54 @@
+#!/bin/bash
+# Steam in the Plasma desktop session: gamepad UI args, virtual keyboard, autostart.
+# Sourced by setup-gamescope-boot.sh; not meant to be run on its own.
+
+setup_steam_desktop() {
+    # Step 8b: always runs.
+    info "Applying non-optional SteamOS environment & keyboard fixes..."
+    local steam_env_dir gs_env systemd_user_dir
+    steam_env_dir="$HOME/.config/environment.d"
+    mkdir -p "$steam_env_dir"
+
+    # 1. Force Gamescope Steam to boot into Deck mode and load Deck glyphs
+    echo "STEAM_GAMEPADUI_ARGS=\"-gamepadui -steamos3\"" > "$steam_env_dir/99-gamescope-steam-glyphs.conf"
+    gs_env="$HOME/.config/gamescope-session/environment"
+    if [[ -d "$HOME/.config/gamescope-session" ]]; then
+        touch "$gs_env"
+        sed -i '/^STEAM_GAMEPADUI_ARGS=/d' "$gs_env"
+        echo "STEAM_GAMEPADUI_ARGS=\"-gamepadui -steamos3\"" >> "$gs_env"
+    fi
+
+    # 2. Prevent KDE Wayland from blocking the virtual keyboard overlay
+    echo "KWIN_IM_SHOW_ALWAYS=1" > "$steam_env_dir/99-kde-virtual-keyboard.conf"
+
+    # 3. Setup robust background systemd service for Steam autostart on Desktop Mode
+    systemd_user_dir="$HOME/.config/systemd/user"
+    mkdir -p "$systemd_user_dir"
+
+    cat << 'EOF' > "$systemd_user_dir/steam-desktop-autostart.service"
+[Unit]
+Description=Steam Background Autostart for Virtual Keyboard
+After=graphical-session.target
+PartOf=graphical-session.target
+
+[Service]
+Type=simple
+# Only in the Plasma desktop session; gamescope-session starts its own Steam.
+ExecCondition=/bin/sh -c '[ "$XDG_CURRENT_DESKTOP" = KDE ]'
+ExecStart=/usr/bin/steam -silent
+Restart=on-failure
+RestartSec=5
+
+[Install]
+WantedBy=graphical-session.target
+EOF
+
+    # Clean out old .desktop shortcut so they don't fight
+    rm -f "$HOME/.config/autostart/steam.desktop"
+
+    # Reload user systemd context and activate the background loop
+    systemctl --user daemon-reload
+    systemctl --user enable --now steam-desktop-autostart.service
+
+    ok "Steam UI parameters and keyboard autostart deployed successfully."
+}