refactor: decouple commands from god-module lib.rs

[vibemasshq-dev]

Hi there! 👋 I've been really enjoying the Downpour application and wanted to contribute to the codebase by helping organize the core architecture.

What does this PR do?
I actually ran your codebase through an AI CTO tool I'm building called VibeMass, and its Architecture Agent flagged src-tauri/src/lib.rs as a "God-module" bottleneck because it handles the registration of 19 different Tauri commands along with extensive state wiring.

I took a look, and the AI was spot on! It was definitely getting a bit crowded in there, so I rolled up my sleeves and refactored that structure to make the codebase much easier for you to maintain and scale.

Specifically, I have:

• Created a dedicated src-tauri/src/commands/ module directory.
• Extracted and grouped related command logic into dedicated sub-modules (download.rs, queue.rs, media.rs, settings.rs, and system.rs).
• Streamlined lib.rs so its primary responsibility is now just importing and exposing these granular modules.

Validation:
I've strictly followed the CONTRIBUTING.md guidelines to ensure this doesn't break anything:

• ✅ cargo fmt applied
• ✅ cargo clippy -- -D warnings passes perfectly
• ✅ cargo test passes (197 unit tests, 10 integration tests)
• ✅ node extension/filter.property.test.js passes
• ✅ Full production native build tested successfully

Thank you for building such an awesome project! I hope this architectural improvement (and my AI's catch) makes future development even smoother.