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.

docs/ROADMAP.md

@@ -404,6 +404,11 @@ Design/decision records and a running log of shipped work (items marked
   decision rather than a drive-by. Proved to fail: dropping snapshot makes it
   name the missing entry. **V3 pending** — that the apps *launch* needs real
   hardware (HARDWARE-QUEUE, Acer M5-481T).
+- ✓ **Fix batch #112 / #106 / #113:** installer disk picker drops fd0/loop/
+  optical/<8 GiB and lists largest-first (OVMF no longer defaults to floppy);
+  `checks.menu-back` guards every internal rofi list emits ↩ Back; offline
+  theme-switch promise scoped to the live-pinned (default) theme with a clear
+  error when a non-pinned preset needs network.
 - ✓ **Install fix batch (#123–#126):** (123) HM pre-activate creates
   `/run/user/$UID` + `dbus-run-session` so dconf no longer aborts the bake;
   (124) main-built ISOs seed `?ref=main` and installer actually rewrites