fix: Replace the question flow with a menu that turns components on and off

- Menu detects what is on and applies only the changes: SteamOS
  conversion, SteamOS theme, Steam Deck/Machine icons, single user mode,
  and Steam Machine support (only on Fremont hardware)
- Everything is reversible: KDE settings go through an undo journal
  (lib/state.sh), system files are restored from their backups
- Steam Machine support: LED driver, udev rule giving the user the LED
  files, steamos-manager for Steam's hardware settings; OpenRGB dropped
- Edit Plasma files only while plasmashell is stopped so it can't
  overwrite them
- README and AGENTS.md describe the menu, the component model and the
  test environment

lib/state.sh

@@ -0,0 +1,79 @@
+#!/bin/bash
+# Undo journal for per-user KDE settings, so every component can be turned
+# off again exactly: the first time a component changes a key, the key's
+# previous value is recorded, and reverting the component puts it back (or
+# deletes the key if it didn't exist before).
+# Sourced by setup-gamescope-boot.sh; not meant to be run on its own.
+
+STATE_DIR="${XDG_STATE_HOME:-$HOME/.local/state}/cachyos-gamescope-boot"
+UNSET=$'\x01'
+
+kset() {
+    # kset <component> <file> <group[|group...]> <key> <value>
+    # kwriteconfig6, recording the old value in the component's journal.
+    # <value> "--delete" removes the key.
+    local component="$1" file="$2" groups="$3" key="$4" value="$5"
+    local journal="$STATE_DIR/$component.journal" old id
+    local -a gargs=() g
+    local groups_item
+    IFS='|' read -ra g <<< "$groups"
+    for groups_item in "${g[@]}"; do gargs+=(--group "$groups_item"); done
+
+    mkdir -p "$STATE_DIR"
+    id="$(printf '%s\t%s\t%s' "$file" "$groups" "$key")"
+    if ! grep -qxF -- "$id" <(cut -f1-3 "$journal" 2>/dev/null); then
+        old="$(kreadconfig6 --file "$file" "${gargs[@]}" --key "$key" --default "$UNSET")"
+        printf '%s\t%s\n' "$id" "$(printf '%s' "$old" | base64 -w0)" >> "$journal"
+    fi
+
+    if [[ "$value" == "--delete" ]]; then
+        kwriteconfig6 --file "$file" "${gargs[@]}" --key "$key" --delete
+    else
+        kwriteconfig6 --file "$file" "${gargs[@]}" --key "$key" "$value"
+    fi
+}
+
+krevert() {
+    # krevert <component>: restore every key the component changed, newest
+    # first, then forget the journal.
+    local component="$1"
+    local journal="$STATE_DIR/$component.journal"
+    local file groups key old groups_item
+    local -a g
+    [[ -f "$journal" ]] || return 0
+    while IFS=$'\t' read -r file groups key old; do
+        local -a gargs=()
+        IFS='|' read -ra g <<< "$groups"
+        for groups_item in "${g[@]}"; do gargs+=(--group "$groups_item"); done
+        old="$(printf '%s' "$old" | base64 -d)"
+        if [[ "$old" == "$UNSET" ]]; then
+            kwriteconfig6 --file "$file" "${gargs[@]}" --key "$key" --delete
+        else
+            kwriteconfig6 --file "$file" "${gargs[@]}" --key "$key" "$old"
+        fi
+    done < <(tac "$journal")
+    rm -f "$journal"
+}
+
+has_journal() {
+    [[ -s "$STATE_DIR/$1.journal" ]]
+}
+
+state_set() {
+    # state_set <component> <name> <value>: remember a value for later.
+    mkdir -p "$STATE_DIR"
+    kwriteconfig6 --file "$STATE_DIR/$1.state" --group State --key "$2" "$3"
+}
+
+state_get() {
+    # state_get <component> <name> [default]
+    kreadconfig6 --file "$STATE_DIR/$1.state" --group State --key "$2" --default "${3:-}"
+}
+
+state_clear() {
+    rm -f "$STATE_DIR/$1.state"
+}
+
+current_display_manager() {
+    systemctl show -p Id --value display-manager 2>/dev/null | sed 's/\.service$//'
+}