fix(frontend): Remove hardcoded bypass of billing feature flag (#11762)

## Summary

Fixes a critical security issue where the billing button in the settings
sidebar was always visible to all users, bypassing the
`ENABLE_PLATFORM_PAYMENT` feature flag.

## Changes 🏗️

- Removed hardcoded `|| true` condition in
`frontend/src/app/(platform)/profile/(user)/layout.tsx:32` that was
bypassing the feature flag check
- The billing button is now properly gated by the
`ENABLE_PLATFORM_PAYMENT` feature flag as intended

## Root Cause

The `|| true` was accidentally left in commit
3dbc03e488 (PR #11617 - OAuth API & Single
Sign-On) from December 19, 2025. It was likely added temporarily during
development/testing to always show the billing button, but was not
removed before merging.

## Test Plan

1. Verify feature flag is set to disabled in LaunchDarkly
2. Navigate to settings page (`/profile/settings`)
3. Confirm billing button is NOT visible in the sidebar
4. Enable feature flag in LaunchDarkly
5. Refresh page and confirm billing button IS now visible
6. Verify billing page (`/profile/credits`) is still accessible via
direct URL when feature flag is disabled

## Checklist 📋

### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan

Fixes SECRT-1791

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* The Billing link in the profile sidebar now respects the payment
feature flag configuration and will only display when payment
functionality is enabled.

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settings.</sub>

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

autogpt_platform/frontend/src/app/(platform)/profile/(user)/layout.tsx

@@ -29,7 +29,7 @@ export default function Layout({ children }: { children: React.ReactNode }) {
           href: "/profile/dashboard",
           icon: <StorefrontIcon className="size-5" />,
         },
-        ...(isPaymentEnabled || true
+        ...(isPaymentEnabled
           ? [
               {
                 text: "Billing",