Fix frontend tsconfig for TypeScript 6.0

TypeScript 6.0 requires an explicit `rootDir` when linked-plugin
symlinks push the common source directory outside the project (TS5011),
and type declarations even for side-effect imports such as
`moment/locale/*` (TS2882). Angular 22 also rejects
`extendedDiagnostics` while `strictTemplates` is off (NG4003).

frontend/src/typings/moment-locales.d.ts

@@ -0,0 +1,3 @@
+// moment ships its locale files without type declarations.
+// TypeScript 6.0 requires declarations even for side-effect imports (TS2882).
+declare module 'moment/locale/*';

frontend/tsconfig.app.json

@@ -2,6 +2,7 @@
   "extends": "./tsconfig.json",
   "compilerOptions": {
     "outDir": "./out-tsc/app",
+    "rootDir": "..",
     "jsx": "react-jsx",
     "types": []
   },
@@ -21,12 +22,6 @@
     "src/stimulus/controllers/**/*.spec.ts"
   ],
   "angularCompilerOptions": {
-    "extendedDiagnostics": {
-      "checks": {
-        "nullishCoalescingNotNullable": "suppress",
-        "optionalChainNotNullable": "suppress"
-      }
-    },
     "strictTemplates": false
   },
   "types": [

frontend/tsconfig.spec.json

@@ -20,12 +20,6 @@
     "src/app/**/*.module.ts"
   ],
   "angularCompilerOptions": {
-    "extendedDiagnostics": {
-      "checks": {
-        "nullishCoalescingNotNullable": "suppress",
-        "optionalChainNotNullable": "suppress"
-      }
-    },
     "strictTemplates": false
   }
 }