feat: close #54 V2 install safety, #33 rofi scrollbar, #43 MOTD tip

#54: permanent checks.installer-safety; test-install-safety harness with
offline compose-fail, LUKS-signature disk warn, and HM-hint FORCE paths.
#33: enable themed rofi scrollbars (generated + all whole-swap .rasi).
#43: MOTD + first-boot tip for System › Firmware (V3 LVFS stays hw-queue).

V2: compose-fail/disk-warn/hm-hint logs green offline; theme-shot
boreal+summer-night menus OK. V0 flake/installer-safety.

modules/nixos/default.nix

@@ -57,6 +57,7 @@ in
         home-update                         apply home/theme changes (no sudo)
         nomarchy-theme-sync apply <theme>   switch the whole palette
         SUPER+?                             keybindings cheatsheet
+        SUPER+M → System › Firmware         check LVFS firmware updates (fwupd)
     '';
 
     # Unfree allowed distro-wide: pragmatic-desktop territory (vendor