Encode sign-in mail as base64 so webmail cannot mangle the verify token.

Quoted-printable still rewrote token= as token=3D in the raw MIME, which some clients never decode. Distinguish a missing token from a real expiry, and keep links valid for 60 minutes.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/components/LoginForm.test.tsx

@@ -240,6 +240,14 @@ describe("LoginForm", () => {
     ).toBeInTheDocument();
   });
 
+  it("shows URL-driven error for invalid_link", () => {
+    navMock.searchParams = new URLSearchParams("error=invalid_link");
+    renderLoginForm();
+    expect(
+      screen.getByText(/that sign-in link is not valid/i),
+    ).toBeInTheDocument();
+  });
+
   it("calls router.replace to clear error query when user types (full-page /login)", async () => {
     const user = userEvent.setup();
     navMock.pathname = "/login";