refactor(shared): extract reused path and normalization helpers

src/browser/safe-filename.ts

@@ -0,0 +1,26 @@
+import path from "node:path";
+
+export function sanitizeUntrustedFileName(fileName: string, fallbackName: string): string {
+  const trimmed = String(fileName ?? "").trim();
+  if (!trimmed) {
+    return fallbackName;
+  }
+  let base = path.posix.basename(trimmed);
+  base = path.win32.basename(base);
+  let cleaned = "";
+  for (let i = 0; i < base.length; i++) {
+    const code = base.charCodeAt(i);
+    if (code < 0x20 || code === 0x7f) {
+      continue;
+    }
+    cleaned += base[i];
+  }
+  base = cleaned.trim();
+  if (!base || base === "." || base === "..") {
+    return fallbackName;
+  }
+  if (base.length > 200) {
+    base = base.slice(0, 200);
+  }
+  return base;
+}