fix: disk picker fd0 (#112), menu Back guard (#106), offline theme (#113)

- #112: list_installable_disks drops fd/loop/sr/zram/<8GiB, largest-first
  so OVMF no longer offers /dev/fd0 as the default wipe target
- #106: tools/check-menu-back.py + checks.menu-back (proved to fail)
- #113: offline theme-switch contract documented (default/pinned only);
  theme-sync run_switch adds offline-oriented hint when no network

Verified: V0 flake check; installer-safety; menu-back; py_compile.

pkgs/nomarchy-theme-sync/nomarchy-theme-sync.py

@@ -370,9 +370,23 @@ def run_switch() -> None:
             "Rebuild FAILED — scroll up for last lines; run nomarchy-doctor",
             urgency="critical",
         )
+        # Live ISO pins only the *default* theme's HM generation. Applying
+        # another preset offline tries to build the world from source (#113).
+        offline_hint = ""
+        try:
+            import socket
+            socket.create_connection(("1.1.1.1", 53), timeout=1.5).close()
+        except OSError:
+            offline_hint = (
+                " Offline? Only the live ISO's already-pinned theme is "
+                "guaranteed without a network — other themes need builds "
+                "or a binary cache. Connect and retry, or apply the default "
+                "theme (boreal)."
+            )
         die(
             "rebuild failed (state already written; fix and re-run). "
-            "Diagnose: nomarchy-doctor. Recovery: docs/RECOVERY.md"
+            "Diagnose: nomarchy-doctor. Recovery: docs/RECOVERY.md."
+            + offline_hint
         )
     notify("Changes applied ✓")
     # Waybar runs from Hyprland's exec-once (not a systemd unit HM would