TypeScript: use strict mode

frontend/tsconfig.json

@@ -1,6 +1,10 @@
 {
   "extends": "@tsconfig/svelte/tsconfig.json",
+  "compilerOptions": {
+    // "target": "ES2019"
+    "strict": true
+  },
 
   "include": ["src/**/*"],
   "exclude": ["node_modules/*", "__sapper__/*", "public/*"],
-}
+}