fix(ci): make root lint script a no-op-when-absent gate

pnpm -r lint exited 1 (ERR_PNPM_RECURSIVE_RUN_NO_SCRIPT) because no package
defines a lint script yet, failing the CI fast-checks job. Switch to
pnpm -r --if-present lint so it exits 0 today and gates automatically once a
lint script is added. (Lint tooling wiring remains out of scope.)

package.json

@@ -8,7 +8,7 @@
     "build": "pnpm --filter @familysync/api build && pnpm --filter @familysync/pwa build",
     "test": "pnpm --filter @familysync/api test",
     "test:e2e": "pnpm --filter @familysync/pwa test:e2e",
-    "lint": "pnpm -r lint",
+    "lint": "pnpm -r --if-present lint",
     "typecheck": "pnpm -r typecheck"
   }
 }