functions: move lint to Biome and fix every warning

.github/workflows/build.yml

@@ -57,6 +57,15 @@ jobs:
             - name: functions npm install (ci)
               working-directory: ./functions
               run: npm ci
+            # Fails on warnings too, not only on errors (info-level
+            # suggestions do not fail).
+            - name: lint functions/
+              working-directory: ./functions
+              run: npm run lint
+            # The build excludes the specs and test utilities.
+            - name: typecheck functions/ specs
+              working-directory: ./functions
+              run: npm run typecheck
             - name: npm test (Firestore rules and functions/)
               run: |
                   npm run test

functions/biome.json

@@ -0,0 +1,35 @@
+{
+    "$schema": "https://biomejs.dev/schemas/2.6.4/schema.json",
+    "root": true,
+    "vcs": {
+        "enabled": true,
+        "clientKind": "git",
+        "useIgnoreFile": true
+    },
+    "files": {
+        "includes": ["**", "!lib"]
+    },
+    "formatter": {
+        "enabled": true,
+        "indentStyle": "space",
+        "indentWidth": 4,
+        "lineWidth": 80
+    },
+    "javascript": {
+        "formatter": {
+            "quoteStyle": "single",
+            "semicolons": "asNeeded",
+            "trailingCommas": "es5",
+            "arrowParentheses": "always"
+        }
+    },
+    "assist": {
+        "enabled": false
+    },
+    "linter": {
+        "enabled": true,
+        "rules": {
+            "preset": "recommended"
+        }
+    }
+}