Explicitly set tabIndex for log in buttons, since Safari does not interpret them as "interactive elements" in the same way it does input fields; clean up, test (#37019)

<!-- Add the related story/sub-task/bug number, like Resolves #123, or
remove if NA -->
**Related issue:** Resolves #36735 (see
https://fleetdm.slack.com/archives/C019WG4GH0A/p1765304390750399?thread_ts=1765303643.855049&cid=C019WG4GH0A)

Safari now has same tab order without having to hold option button while
tabbing:

![ezgif-758724f23b5b7550](https://github.com/user-attachments/assets/9ba83634-9211-4c61-9798-d84a1ebb9a15)

- Confirmed order is maintained on Firefox and Chrome
- Cleaned up inaccurate emptiness checks and unnecessary wrappers
- Test

- [x] QA'd all new/changed functionality manually
- [x] Added tests 
- [x] Confirmed that the fix is not expected to adversely impact load
test results

frontend/components/forms/LoginForm/LoginForm.tests.tsx

@@ -140,4 +140,29 @@ describe("LoginForm - component", () => {
       password,
     });
   });
+  it("tabs in the expected order", async () => {
+    const { user } = renderWithSetup(
+      <LoginForm
+        handleSubmit={submitSpy}
+        isSubmitting={false}
+        pendingEmail={false}
+        ssoSettings={{
+          sso_enabled: true,
+          idp_name: "Test IdP",
+        }}
+      />
+    );
+
+    expect(screen.getByPlaceholderText("Email")).toHaveFocus();
+    await user.tab();
+    expect(screen.getByPlaceholderText("Password")).toHaveFocus();
+    await user.tab();
+    expect(screen.getByText("Log in").parentElement).toHaveFocus();
+    await user.tab();
+    expect(
+      screen.getByText("Sign in with Test IdP").parentElement
+    ).toHaveFocus();
+    await user.tab();
+    expect(screen.getByText("Forgot password?")).toHaveFocus();
+  });
 });

frontend/components/forms/LoginForm/LoginForm.tsx

@@ -84,28 +84,28 @@ const LoginForm = ({
 
   const showLegendWithImage = () => {
     let legend = "Single sign-on";
-    if (idpName !== "") {
+    if (idpName) {
       legend = `Sign in with ${idpName}`;
     }
 
     return (
-      <div>
+      <>
         <img
           src={imageURL}
           alt={idpName}
           className={`${baseClass}__sso-image`}
         />
         <span className={`${baseClass}__sso-legend`}>{legend}</span>
-      </div>
+      </>
     );
   };
 
   const renderSingleSignOnButton = () => {
     let legend: string | JSX.Element = "Single sign-on";
-    if (idpName !== "") {
+    if (idpName) {
       legend = `Sign in with ${idpName}`;
     }
-    if (imageURL !== "") {
+    if (imageURL) {
       legend = showLegendWithImage();
     }
 
@@ -116,8 +116,9 @@ const LoginForm = ({
         title="Single sign-on"
         variant="inverse"
         onClick={handleSSOSignOn}
+        tabIndex={0}
       >
-        <div>{legend}</div>
+        {legend}
       </Button>
     );
   };
@@ -183,6 +184,7 @@ const LoginForm = ({
             className={`${baseClass}__login-btn`}
             isLoading={isSubmitting}
             type="submit"
+            tabIndex={0}
           >
             Log in
           </Button>