quak backup refuses to run while another backup of the same directory runs, exit 2 (closes #169)

lib.backup() takes a lock, backup.lock in its download directory, made
with proper-lockfile, before its refresh, and removes it when it ends. A
second backup of the directory fails at once with an error naming it;
quak backup prints it as one line and exits 2. A lock untouched for 10
seconds, left by a killed run, is taken over.

Deviation: yarn.lock was regenerated by yarn add in the pinned node image.
Judgement call: a run failing at its refresh leaves the directory, empty.
Judgement call: a lock removed mid-run stops that run with an uncaught
error, the library's default.

Model: opus-5-5

package.json

@@ -36,6 +36,7 @@
     "devDependencies": {
         "@eslint/js": "9.38.0",
         "@types/node": "22.18.13",
+        "@types/proper-lockfile": "4.1.4",
         "eslint": "9.38.0",
         "prettier": "3.8.1",
         "typescript": "5.9.3",
@@ -50,6 +51,7 @@
         "fast-srp-hap": "2.0.4",
         "fflate": "0.8.3",
         "jpeg-js": "0.4.4",
-        "libsodium-wrappers-sumo": "0.8.4"
+        "libsodium-wrappers-sumo": "0.8.4",
+        "proper-lockfile": "4.1.2"
     }
 }