feat(subtitles): implement subtitle translation proxy support

[nutine]

Adds support for on-the-fly subtitle translation using an external translation proxy server.

Instead of embedding heavy translation logic, vendor APIs, or keys into the app, this keeps the client completely decoupled:

• Users can set a translation proxy URL in Player Subtitles settings.
• Selecting "Translate subtitles" in the player lets you pick a target language.
• The app builds a proxied URL (<proxy>/translate?url=<sub_url>&target=<lang>) and passes it directly to the player as a standard WebVTT subtitle track.
• The proxy server downloads the subtitle, translates it, and streams back WebVTT.

An open-source reference proxy implementation is available here:
https://github.com/nutine/cloudstream-subtitle-proxy

Tested with local and remote proxy servers on Android 13.

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AI Policy disclosure: An LLM was used to assist in drafting boilerplate, but the architecture, code refactoring, and integration were implemented, verified, and tested manually.

[Bnyro]

Please read the AI Policy. You didn't even write the PR description yourself...

I also don't think that this should be implemented directly on the client, that looks extremely ugly. The only solid way to implement this if there's a dedicated server / proxy that can translate subtitles so that the proxied URL can just be passed to the player instead of the original subtitle URL.

[nutine]

You're completely right about decoupling the client from direct LLM logic — putting vendor API calls and prompt batching directly into the Android app was messy.

I've refactored this PR according to your suggestion:

1. The client now only knows about a configurable subtitle translation proxy URL (in Subtitles settings).
2. When a user requests translation in the player, it simply builds the proxied URL (<proxy>/translate?url=<sub_url>&target=<lang>) and hands it to addAndSelectSubtitles as a regular WebVTT track.
3. The client diff is now minimal (~140 lines total, no LLM SDKs, no batching or parsing hacks in the player).
4. I've also published a reference open-source proxy implementation here that anyone can run/host: https://github.com/nutine/cloudstream-subtitle-proxy

Also updated the description and added the AI disclosure per the policy. Let me know if this looks cleaner!