feat: add insta snapshot testing for TUI components

- Add insta dev-dependency for visual regression testing
- Create lib.rs to expose modules for integration tests
- Add snapshot tests for progress_bar, topbar, library, help modal
- Add unit tests for LibraryState navigation
- Move all tests to tests/ directory (proper Rust convention)
- Make build.rs skip proto compilation when protoc unavailable
- Add docs/testing-possible-solutions.md with testing strategies

Cargo.toml

@@ -15,5 +15,8 @@ tonic = "0.12"
 prost = "0.13"
 tokio = { version = "1", features = ["rt-multi-thread", "macros", "sync", "time"] }
 
+[dev-dependencies]
+insta = "1.40.0"
+
 [build-dependencies]
 tonic-build = "0.12"