lib(renderer): add safeJson helper; migrate 6 JSON.parse try/catch call sites

src/renderer/lib/safe-json.ts

@@ -0,0 +1,8 @@
+export function safeJson<T>(text: string | null | undefined, fallback: T): T {
+  if (text == null || text === "") return fallback;
+  try {
+    return JSON.parse(text) as T;
+  } catch {
+    return fallback;
+  }
+}