fix: improve whitelist feature flag comments for clarity (#6457)

## Summary

This PR improves code comments to accurately describe the whitelist
feature flag implementation logic.

## Changes

- Updated comments in `router.ts` and `UserCheckView.vue` to clarify
that the feature flag is checked first before user status
- Removed unreachable comment after return statement in
`UserCheckView.vue`
- Comments now accurately reflect the actual code execution order

## Technical Details

The logic flow remains unchanged:
1. Check `require_whitelist` feature flag first (defaults to `true`)
2. If flag is `true` AND user status is not `'active'`, redirect to
waitlist
3. If flag is `false`, allow all users to proceed regardless of status

## Testing

No functional changes - only comment improvements for better code
maintainability.

🤖 Generated with [Claude Code](https://claude.ai/code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-6457-fix-improve-whitelist-feature-flag-comments-for-clarity-29c6d73d365081cf8a59d662118f7243)
by [Unito](https://www.unito.io)

Co-authored-by: Claude <noreply@anthropic.com>

global.d.ts

@@ -8,6 +8,7 @@ declare const __USE_PROD_CONFIG__: boolean
 interface Window {
   __CONFIG__: {
     mixpanel_token?: string
+    require_whitelist?: boolean
     subscription_required?: boolean
     server_health_alert?: {
       message: string