Custom mpv configuration support for subtitle styling

[imheinrichlunge]

Is your feature request related to a problem? Please describe.
When using the mpv player, there is currently no way to customize the appearance of subtitles. While the default player relies on the native Android "Closed Caption" system settings, these settings are very limited and look horrible. This leaves users with no way to adjust font size, color, or margins when using the mpv toggle, which is the preferred playback method for many.

Describe the solution you'd like
I would like an option within Findroid's player settings to pass custom parameters to the mpv player, similar to how the standalone mpv Android app works. This could be implemented in two ways:

1. Direct mpv.conf text input: A text field in the settings where users can manually input configuration lines (e.g., --sub-font-size=55).
2. GUI for common sub settings: An user interface with sliders and color pickers that generates these parameters.

At a minimum, a simple text edit field (like the one in the mpv app from the Play Store) would allow power users to fully customize their experience.

Describe alternatives you've considered

• Android closed caption settings: These are ugly, limited, and don't provide the level of control needed for high-quality media playback.
• External Players: I have intentionally chosen Findroid to avoid relying on external players, as I did with the official Jellyfin client. I want to maintain the seamless integration, watch-status tracking, and "all-in-one" feel that Findroid provides, but with the subtitle legibility of a dedicated player.

Additional context
Being able to specify a custom fonts directory is a major part of this request. For reference, this is the configuration I use in the standalone mpv app to achieve readable, high-quality subtitles:

--sub-color="#fafaf5"
--sub-border-color="#000000"
--sub-border-size=2
--sub-font-size=55
--sub-font="Satoshi Black"
--sub-fonts-dir="/storage/emulated/0/Android/media/is.xyz.mpv/Fonts"
--sub-margin-y=88

[edifris]

Agree.
For the meantime you can edit it manually by editing /android/data/dev.jdtech.jellyfin/files/mpv/mpv.conf