feat: implement native 3D virtual surround, video zoom/crop ratios, and precision playback speed slider

[adityaonx]

• Added 3D Audio Virtual Surround with DSP parameters and UI presets
• Implemented Screen Size / Crop menu with Zoom slider and aspect ratio overrides
• Upgraded Playback Speed menu with a precise ±0.1x adjustment slider
• Integrated comprehensive keyboard shortcuts for all new features
• Added technical architecture and feature documentation.

[FengZeng]

Thank you very much for the PR! There are several great ideas here, especially using a slider for precise playback speed adjustment, it feels very intuitive. The zoom feature and the audio-enhancement settings based on mpv audio filters are also both very valuable.

However, this PR covers quite a lot of ground. I’d suggest splitting it into several smaller, more focused PRs—for example: playback speed and shortcuts, zoom/cropping, audio filters and settings UI, and documentation updates. That would make them easier to review, test, and maintain.

I also noticed a few issues that would be good to address first:

• After adjusting playback speed with the slider, the player controls disappear immediately, making continuous adjustments difficult.
• Crop presets invoke an unregistered Tauri command: mpv_set_property.
• Zoom and playback-speed shortcuts are not synchronized with the current UI/mpv state. If a value is changed through the menu first, using the shortcuts afterwards starts calculating from 1.0 again.
• The aspect-ratio presets calculate zoom from a hard-coded 16:9 base ratio, so they don’t produce the intended crop for non-16:9 videos.

If you don’t have time to work on these, that’s totally fine. I may use this PR as a reference and add the related features myself later. But if you’d like to continue with it, I’d be happy to review smaller, focused PRs for these features.

[FengZeng]

Continuous playback speed control added in 6948d7e