fix(install): #54 safety guards + recovery hint

Offline compose-lock failure now fail-closes (reusing the cache.nixos.org
probe) instead of falling back to a network `nix flake lock`; a disk-review
warn fires when the target already carries Windows/BitLocker/NTFS/LUKS
(before the wipe); account password requires ≥8 chars like LUKS; on HM
pre-activate failure the installer drops a NOMARCHY-DESKTOP-NOT-THEMED.txt
recovery hint in the target home so first login still shows the fix.

Worktree agent; diff reviewed + cherry-picked. V0 flake check + V1
nomarchy-install build. Password ≥8 is V0-complete; the offline/NTFS/
pre-activate scenarios are V2-pending (crafted install runs).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pkgs/nomarchy-install/nomarchy-install.sh

@@ -89,7 +89,9 @@ export NIX_CONFIG="flake-registry = $SHARE/registry.json"
 # substituter resolves to the TARGET store (i.e. itself), so without
 # this nothing flows from the ISO and nix bootstraps gcc from source.
 NIXOS_INSTALL_OPTS=()
+OFFLINE=false
 if ! timeout 3 bash -c '</dev/tcp/cache.nixos.org/443' 2>/dev/null; then
+    OFFLINE=true
     info "No network — substituting from the ISO store only."
     NIX_CONFIG+=$'\nsubstituters =\nextra-substituters = daemon?trusted=1\nbuilders ='
     # Must ALSO go through nixos-install as a flag: it passes its own
@@ -122,6 +124,15 @@ fi
 [[ -b "$TARGET_DISK" ]] || fail "$TARGET_DISK is not a block device."
 info "Target: $TARGET_DISK"
 
+# Single whole-disk install only (no dual-boot path) — if the chosen disk
+# already carries a recognizable OS/filesystem signature, call it out
+# explicitly before the pre-wipe below destroys it.
+existing_sig="$(lsblk -no FSTYPE,LABEL "$TARGET_DISK" 2>/dev/null || true
+                blkid "$TARGET_DISK"* 2>/dev/null || true)"
+if grep -qiE 'ntfs|bitlocker|microsoft|crypto_luks' <<< "$existing_sig"; then
+    warn "$TARGET_DISK has existing data (Windows/BitLocker/NTFS or LUKS) — it will be destroyed."
+fi
+
 # ─── Encryption ─────────────────────────────────────────────────────────
 section "Disk encryption"
 
@@ -181,8 +192,8 @@ else
         warn "Invalid username (lowercase letters, digits, - and _)."
     done
     while true; do
-        PASSWORD=$(gum input --password --placeholder "password for $USERNAME")
-        [[ -n "$PASSWORD" ]] || { warn "Empty password."; continue; }
+        PASSWORD=$(gum input --password --placeholder "password for $USERNAME (min 8 chars)")
+        [[ ${#PASSWORD} -ge 8 ]] || { warn "Too short (min 8 chars)."; continue; }
         p2=$(gum input --password --placeholder "repeat password")
         [[ "$PASSWORD" == "$p2" ]] && break
         warn "Passwords don't match."
@@ -445,6 +456,9 @@ PYJSON
 # `nix flake update` on the installed machine re-resolves normally).
 if ! python3 "$SHARE/compose-lock.py" "$SHARE/flake.lock" "$FLAKE_DIR/flake.lock" \
         "$NOMARCHY_LOCKED_JSON" "$NOMARCHY_ORIGINAL_JSON"; then
+    if [[ "$OFFLINE" == true ]]; then
+        fail "Offline lock composition failed and there is no network to fall back to — cannot finish an offline install."
+    fi
     warn "Offline lock composition failed — resolving over the network."
     (cd "$FLAKE_DIR" && nix --extra-experimental-features "nix-command flakes" flake lock)
 fi
@@ -577,6 +591,20 @@ else
     warn "on the installed system); after first login run:"
     warn "  home-manager switch --flake ~/.nomarchy -b bak"
     tail -n 5 /mnt/var/log/nomarchy-hm-preactivate.log 2>/dev/null || true
+    # The live session (and this warning) ends with this install — drop a
+    # durable hint on the TARGET so the fix still surfaces on first login.
+    # Numeric ids: the account doesn't exist in the live environment yet
+    # (same reasoning as the $FLAKE_DIR chown above).
+    hint_file="/mnt/home/$USERNAME/NOMARCHY-DESKTOP-NOT-THEMED.txt"
+    cat > "$hint_file" <<HINT
+Desktop pre-activation failed during install — see
+/var/log/nomarchy-hm-preactivate.log for details. Finish it with:
+
+    home-manager switch --flake ~/.nomarchy -b bak
+
+(delete this file once done)
+HINT
+    chown 1000:100 "$hint_file"
 fi
 rm -f /mnt/root/nomarchy-hm-activate.sh