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.

themes/summer-day/rofi.rasi

@@ -73,6 +73,7 @@ entry {
 }
 
 listview {
+    scrollbar: true;
     background-color: transparent;
     columns: 1;
     lines: 8;
@@ -131,9 +132,9 @@ element-icon selected {
 }
 
 scrollbar {
-    width: 0px;
-    background-color: transparent;
-    handle-width: 0px;
-    handle-color: transparent;
+    width: 4px;
+    handle-width: 4px;
+    handle-color: @yellow;
+    background-color: @bg0;
     border: 0px;
 }