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

build.rs

@@ -1,17 +1,22 @@
 fn main() -> Result<(), Box<dyn std::error::Error>> {
     let proto_root = "../music-agregator/proto";
+    let proto_file = format!("{}/music_agregator/v1/music_agregator.proto", proto_root);
+
+    let proto_file_exists = std::path::Path::new(&proto_file).exists();
+    let protoc_available = std::process::Command::new("protoc")
+        .arg("--version")
+        .output()
+        .is_ok();
+
+    if !proto_file_exists || !protoc_available {
+        return Ok(());
+    }
 
     tonic_build::configure()
         .build_server(false)
         .build_client(true)
         .out_dir("src/proto")
-        .compile_protos(
-            &[format!(
-                "{}/music_agregator/v1/music_agregator.proto",
-                proto_root
-            )],
-            &[proto_root],
-        )?;
+        .compile_protos(&[proto_file], &[proto_root])?;
 
     Ok(())
 }