feat(menu): Back on every submenu + working snapshot fallback

Two menu improvements.

Back everywhere: every list menu now ends with a "↩ Back" entry that
returns one level up — power, theme, power-profile, clipboard, files,
capture, not just Tools/System — so you no longer have to Esc out and
reopen to back up. A `back` helper + a shared `BACK` label keep it uniform;
matched exactly so it can't collide with clipboard/filename content. Esc
still quits instantly.

Snapshot fallback: btrfs-assistant 2.2 segfaults on 26.05 (libbtrfsutil
ABI), so `nomarchy-menu snapshot` now launches nomarchy-snapshots instead —
a keyboard-driven snapper browser/restore. snapper is root-only here and
rofi can't run as root under Wayland, so it runs in a terminal via sudo
(one password prompt) and uses fzf to pick: browse/diff (read-only),
restore files (undochange), or roll back (root config only), each behind a
typed-`yes` confirmation — safer than a one-click rofi rollback.
btrfs-assistant stays installed for when nixpkgs fixes it.

Verified: flake check clean; home generation builds and the menu passes
bash -n with all Back + snapshot wiring; nomarchy-snapshots passes
shellcheck (writeShellApplication) in a snapper-enabled system build.
Pending an on-machine check of the restore/rollback paths.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/nixos/default.nix

@@ -325,8 +325,69 @@ in
         nixos-rebuild switch --flake /etc/nixos#default "$@"
       '')
     # The desktop snapshot manager (browse / diff / restore / rollback over
-    # snapper, elevating via polkit) — what `nomarchy-menu snapshot` launches.
-    ++ lib.optional cfg.snapper.enable pkgs.btrfs-assistant;
+    # snapper, elevating via polkit). Kept for when nixpkgs fixes its crash.
+    ++ lib.optional cfg.snapper.enable pkgs.btrfs-assistant
+    # Keyboard-driven snapper browser/restore — the fallback `nomarchy-menu
+    # snapshot` actually launches while btrfs-assistant 2.2 segfaults on 26.05
+    # (libbtrfsutil ABI). Runs as root (snapper is root-only here; the menu
+    # opens it in a terminal via sudo, one password prompt), fzf to pick, with
+    # browse/diff (read-only) and typed-`yes` confirmation before any write.
+    ++ lib.optional cfg.snapper.enable (pkgs.writeShellApplication {
+      name = "nomarchy-snapshots";
+      runtimeInputs = with pkgs; [ snapper fzf gawk gnugrep less coreutils systemd ];
+      text = ''
+        if [ "$(id -u)" -ne 0 ]; then
+          echo "nomarchy-snapshots must run as root (snapper needs it) — use sudo." >&2
+          exit 1
+        fi
+
+        mapfile -t configs < <(snapper list-configs | awk 'NR>2 {print $1}' | grep .)
+        if [ "''${#configs[@]}" -eq 0 ]; then
+          echo "No snapper configs found." >&2; exit 1
+        elif [ "''${#configs[@]}" -eq 1 ]; then
+          config="''${configs[0]}"
+        else
+          config=$(printf '%s\n' "''${configs[@]}" | fzf --prompt="snapper config> ") || exit 0
+        fi
+
+        while :; do
+          snap=$(snapper -c "$config" list \
+            | fzf --header-lines=2 --prompt="[$config] pick a snapshot (Esc quits)> ") || exit 0
+          num=$(awk '{print $1}' <<<"$snap")
+          case "$num" in ""|*[!0-9]*) continue ;; esac
+
+          action=$(printf '%s\n' \
+            "Browse changes since #$num (read-only)" \
+            "Restore changed files to #$num (undochange)" \
+            "Roll the system back to #$num (reboot)" \
+            "↩ Back to the snapshot list" \
+            | fzf --prompt="snapshot #$num> ") || exit 0
+
+          case "$action" in
+            Browse*)
+              snapper -c "$config" status "$num..0" | less -R || true ;;
+            Restore*)
+              read -rp "Revert files in '$config' to snapshot #$num? Type yes to confirm: " ans || continue
+              if [ "$ans" = yes ]; then
+                snapper -c "$config" undochange "$num..0"
+                echo "Files restored. Press enter."; read -r _ || true
+              fi ;;
+            Roll*)
+              if [ "$config" != root ]; then
+                echo "Rollback applies to the 'root' config only; use Restore for '$config'. Press enter."
+                read -r _ || true
+              else
+                read -rp "Roll the SYSTEM back to #$num and REBOOT now? Type yes to confirm: " ans || continue
+                if [ "$ans" = yes ]; then
+                  snapper -c root rollback "$num"
+                  echo "Rolled back — rebooting…"; systemctl reboot
+                fi
+              fi ;;
+            *) continue ;;
+          esac
+        done
+      '';
+    });
 
     # Don't let boot entries fill the ESP over the years.
     boot.loader.systemd-boot.configurationLimit = lib.mkDefault 10;