feat(web): safeRedirect helper for the login \?next= param

Falls back to / for anything that isn't a single-slash-prefixed
relative path. Locks out protocol-relative (//evil.com), absolute
(https://evil.com), and javascript: redirects. 7 tests cover the
full attacker matrix.

apps/web/src/lib/safe-redirect.test.ts

@@ -0,0 +1,43 @@
+import { describe, it, expect } from "vitest";
+import { safeRedirect } from "./safe-redirect";
+
+describe("safeRedirect", () => {
+  it("preserves a relative path that starts with a single slash", () => {
+    expect(safeRedirect("/dashboard")).toBe("/dashboard");
+    expect(safeRedirect("/reminders/new")).toBe("/reminders/new");
+  });
+
+  it("preserves query string and fragment", () => {
+    expect(safeRedirect("/legit?with=params&extra=fine#hash")).toBe(
+      "/legit?with=params&extra=fine#hash",
+    );
+  });
+
+  it("rejects protocol-relative URLs (//evil.com)", () => {
+    expect(safeRedirect("//evil.com")).toBe("/");
+    expect(safeRedirect("//evil.com/dashboard")).toBe("/");
+  });
+
+  it("rejects absolute URLs", () => {
+    expect(safeRedirect("https://evil.com")).toBe("/");
+    expect(safeRedirect("http://evil.com/dashboard")).toBe("/");
+  });
+
+  it("rejects javascript: and data: schemes", () => {
+    expect(safeRedirect("javascript:alert(1)")).toBe("/");
+    expect(safeRedirect("data:text/html,<script>x</script>")).toBe("/");
+  });
+
+  it("falls back to / for empty / null / undefined / whitespace input", () => {
+    expect(safeRedirect("")).toBe("/");
+    expect(safeRedirect(null)).toBe("/");
+    expect(safeRedirect(undefined)).toBe("/");
+    expect(safeRedirect("   ")).toBe("/");
+  });
+
+  it("rejects paths that don't start with / (relative-relative)", () => {
+    expect(safeRedirect("dashboard")).toBe("/");
+    expect(safeRedirect("./dashboard")).toBe("/");
+    expect(safeRedirect("../dashboard")).toBe("/");
+  });
+});

apps/web/src/lib/safe-redirect.ts

@@ -0,0 +1,16 @@
+/**
+ * Returns `next` if it is a safe relative path, otherwise "/".
+ *
+ * Safe means: starts with a single forward slash AND not "//" (which
+ * is a protocol-relative URL, e.g. //evil.com). Anything else falls
+ * back to the root — including empty input, absolute URLs, javascript:
+ * URIs, and relative-relative paths like "dashboard" or "../foo".
+ */
+export function safeRedirect(next: string | null | undefined): string {
+  if (typeof next !== "string") return "/";
+  const s = next.trim();
+  if (s.length < 2) return "/";
+  if (!s.startsWith("/")) return "/";
+  if (s.startsWith("//")) return "/";
+  return s;
+}