[Feature] Show a warning when subscribing to an already added feed

Is your feature request related to a problem? Please describe.

I accidentally subscribed twice to the same RSS feed (same URL) and placed it in the same category. FreshRSS allowed the subscription without any warning, and as a result, I started seeing all articles duplicated in my feed reader. This can be confusing and creates unnecessary clutter.

Use case

Users who manage many feeds might unintentionally try to subscribe again to a feed they already follow — for example, while reorganizing categories or using the subscription form in a rush. Currently, there’s no way to know you’re duplicating a subscription unless you manually check your list or notice the duplicated articles later

Describe the solution you’d like

I’d like FreshRSS to display a clear warning or confirmation message when a user tries to subscribe to a feed that’s already present in their subscriptions (ideally even within another category). For example:

“You are already subscribed to this feed. Do you want to continue?”

At minimum, a passive notification (like a message at the top of the subscription page) would help

Describe alternatives you’ve considered

I currently have to manually check the list of subscribed feeds before adding a new one. This works, but it’s tedious, especially with a long list or when using a mobile device. I also tried relying on feed deduplication, but that doesn’t prevent double subscriptions.

Additional context

No response