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

yarn.lock

@@ -535,6 +535,18 @@
   dependencies:
     undici-types "~6.21.0"
 
+"@types/proper-lockfile@4.1.4":
+  version "4.1.4"
+  resolved "https://registry.yarnpkg.com/@types/proper-lockfile/-/proper-lockfile-4.1.4.tgz#cd9fab92bdb04730c1ada542c356f03620f84008"
+  integrity sha512-uo2ABllncSqg9F1D4nugVl9v93RmjxF6LJzQLMLDdPaXCUIDPeOJ21Gbqi43xNKzBi/WQ0Q0dICqufzQbMjipQ==
+  dependencies:
+    "@types/retry" "*"
+
+"@types/retry@*":
+  version "0.12.5"
+  resolved "https://registry.yarnpkg.com/@types/retry/-/retry-0.12.5.tgz#f090ff4bd8d2e5b940ff270ab39fd5ca1834a07e"
+  integrity sha512-3xSjTp3v03X/lSQLkczaN9UIEwJMoMCA1+Nb5HfbJEQWogdeQIyVtTvxPXDQjZ5zws8rFQfVfRdz03ARihPJgw==
+
 "@typescript-eslint/eslint-plugin@8.46.2":
   version "8.46.2"
   resolved "https://registry.yarnpkg.com/@typescript-eslint/eslint-plugin/-/eslint-plugin-8.46.2.tgz#dc4ab93ee3d7e6c8e38820a0d6c7c93c7183e2dc"
@@ -1140,6 +1152,11 @@ globals@^14.0.0:
   resolved "https://registry.yarnpkg.com/globals/-/globals-14.0.0.tgz#898d7413c29babcf6bafe56fcadded858ada724e"
   integrity sha512-oahGvuMGQlPw/ivIYBjVSrWAfWLBeku5tpPE2fOPLi+WHffIWbuh2tCjhyQhTBPMf5E9jDEH4FOmTYgYwbKwtQ==
 
+graceful-fs@^4.2.4:
+  version "4.2.11"
+  resolved "https://registry.yarnpkg.com/graceful-fs/-/graceful-fs-4.2.11.tgz#4183e4e8bf08bb6e05bbb2f7d2e0c8f712ca40e3"
+  integrity sha512-RbJ5/jmFcNNCcDV5o9eTnBLJ/HszWV0P73bc+Ff4nS/rJj+YaS6IGyiOL0VoBYX+l1Wrl3k63h/KrH+nhJ0XvQ==
+
 graphemer@^1.4.0:
   version "1.4.0"
   resolved "https://registry.yarnpkg.com/graphemer/-/graphemer-1.4.0.tgz#fb2f1d55e0e3a1849aeffc90c4fa0dd53a0e66c6"
@@ -1414,6 +1431,15 @@ prettier@3.8.1:
   resolved "https://registry.yarnpkg.com/prettier/-/prettier-3.8.1.tgz#edf48977cf991558f4fcbd8a3ba6015ba2a3a173"
   integrity sha512-UOnG6LftzbdaHZcKoPFtOcCKztrQ57WkHDeRD9t/PTQtmT0NHSeWWepj6pS0z/N7+08BHFDQVUrfmfMRcZwbMg==
 
+proper-lockfile@4.1.2:
+  version "4.1.2"
+  resolved "https://registry.yarnpkg.com/proper-lockfile/-/proper-lockfile-4.1.2.tgz#c8b9de2af6b2f1601067f98e01ac66baa223141f"
+  integrity sha512-TjNPblN4BwAWMXU8s9AEz4JmQxnD1NNL7bNOY/AKUzyamc379FWASUhc/K1pL2noVb+XmZKLL68cjzLsiOAMaA==
+  dependencies:
+    graceful-fs "^4.2.4"
+    retry "^0.12.0"
+    signal-exit "^3.0.2"
+
 punycode@^2.1.0:
   version "2.3.1"
   resolved "https://registry.yarnpkg.com/punycode/-/punycode-2.3.1.tgz#027422e2faec0b25e1549c3e1bd8309b9133b6e5"
@@ -1429,6 +1455,11 @@ resolve-from@^4.0.0:
   resolved "https://registry.yarnpkg.com/resolve-from/-/resolve-from-4.0.0.tgz#4abcd852ad32dd7baabfe9b40e00a36db5f392e6"
   integrity sha512-pb/MYmXstAkysRFx8piNI1tGFNQIFA3vkE3Gq4EuA1dF6gHp/+vgZqsCGJapvy8N3Q+4o7FwvquPJcnZ7RYy4g==
 
+retry@^0.12.0:
+  version "0.12.0"
+  resolved "https://registry.yarnpkg.com/retry/-/retry-0.12.0.tgz#1b42a6266a21f07421d1b0b54b7dc167b01c013b"
+  integrity sha512-9LkiTwjUh6rT555DtE9rTX+BKByPfrMzEAtnlEtdEwr3Nkffwiihqe2bWADg+OQRjt9gl6ICdmB/ZFDCGAtSow==
+
 reusify@^1.0.4:
   version "1.1.0"
   resolved "https://registry.yarnpkg.com/reusify/-/reusify-1.1.0.tgz#0fe13b9522e1473f51b558ee796e08f11f9b489f"
@@ -1502,6 +1533,11 @@ siginfo@^2.0.0:
   resolved "https://registry.yarnpkg.com/siginfo/-/siginfo-2.0.0.tgz#32e76c70b79724e3bb567cb9d543eb858ccfaf30"
   integrity sha512-ybx0WO1/8bSBLEWXZvEd7gMW3Sn3JFlW3TvX1nREbDLRNQNaeNN8WK0meBwPdAaOI7TtRRRJn/Es1zhrrCHu7g==
 
+signal-exit@^3.0.2:
+  version "3.0.7"
+  resolved "https://registry.yarnpkg.com/signal-exit/-/signal-exit-3.0.7.tgz#a9a1767f8af84155114eaabd73f99273c8f59ad9"
+  integrity sha512-wnD2ZE+l+SPC/uoS0vXeE9L1+0wuaMqKlfz9AMUo38JsyLSBWSFcHR1Rri62LZc12vLr1gb3jl7iwQhgwpAbGQ==
+
 signal-exit@^4.1.0:
   version "4.1.0"
   resolved "https://registry.yarnpkg.com/signal-exit/-/signal-exit-4.1.0.tgz#952188c1cbd546070e2dd20d0f41c0ae0530cb04"