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.

tools/check-install-safety.py

@@ -72,6 +72,25 @@ check(
     "missing online-only compose-lock fallback warn",
 )
 
+# ── 1b. Disk picker: exclude floppy/pseudo/tiny (#112) ─────────────────
+check(
+    "list_installable_disks" in script,
+    "missing list_installable_disks helper (#112)",
+)
+check(
+    "/dev/fd*" in script or r"/dev/fd*" in script,
+    "disk filter must exclude /dev/fd* (floppy — OVMF lists fd0 first)",
+)
+check(
+    "MIN_DISK_BYTES" in script,
+    "disk filter must enforce a minimum size floor",
+)
+check(
+    "8 * 1024 * 1024 * 1024" in script or "8 * 1024**3" in script
+    or "$((8 * 1024 * 1024 * 1024))" in script,
+    "default min disk size should be 8 GiB (catches fd0 and crumbs)",
+)
+
 # ── 2. Disk signature warn ──────────────────────────────────────────────
 sig_re = re.search(
     r"grep -qiE '([^']+)'\s*<<<\s*\"\$existing_sig\"",