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>

docs/ROADMAP.md

@@ -51,6 +51,11 @@ how to override it. Items marked ✓ are shipped.
     one-off — any new feature that earns a menu entry must be placed in the
     right submenu (don't let the root creep back to a flat list), with its
     direct `SUPER+CTRL+<mnemonic>` bind and self-gating as applicable.
+  - ✓ **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 never have to Esc out and reopen. 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.
 - ✓ **Menu modules from rofi plugins:** the old `calc` flow committed the
   expression blind (result only in the *next* menu's `-mesg`) and `qalc -t`
   misparsed common phrasings (`15% of 200` → `rem(15, 1 B)`, the natural-
@@ -454,11 +459,19 @@ how to override it. Items marked ✓ are shipped.
     artifact and will crash on hardware too). The menu wiring is correct and
     the app is installed, but the GUI doesn't open. The snapshot *backend*
     (the actual snapshots) is unaffected. Fix path: a nixpkgs override aligning
-    its libbtrfsutil, or wait for an upstream bump; meanwhile the menu entry
-    is a no-op when the binary crashes. Fallback if it lingers: a rofi-based
-    snapshot menu (no btrfs-assistant dependency).
-  - Remaining (optional): a keyboard-driven rofi browse for quick glances;
-    boot-from-snapshot needs a systemd-boot equivalent of grub-btrfs.
+    its libbtrfsutil, or wait for an upstream bump. ✓ **Fallback shipped:**
+    the menu now launches `nomarchy-snapshots` (system-side, gated on
+    `nomarchy.system.snapper`) instead — a keyboard-driven snapper
+    browser/restore with no btrfs-assistant dependency. snapper is root-only
+    here (no `ALLOW_USERS`) 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, which is *safer*
+    than the fat-fingerable one-click rofi rollback this entry originally
+    avoided. btrfs-assistant stays installed for when nixpkgs fixes it.
+    **Pending an on-machine check of the restore/rollback paths.**
+  - Remaining (optional): boot-from-snapshot needs a systemd-boot equivalent
+    of grub-btrfs.
 - **Update awareness:** updates are manual today (`sys-update`/`home-update`);
   add a Waybar indicator / notification when flake inputs are stale or a new
   nixpkgs rev is available (optionally with a pending-change count). Augments,