[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_