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

docs/testing-possible-solutions.md

@@ -0,0 +1,186 @@
+# Testing Strategies for Ratatui TUI Applications
+
+Research summary for testing the ui-agregator TUI application.
+
+## 1. TestBackend + Buffer Assertions (Built-in)
+
+Ratatui provides `TestBackend` - an in-memory terminal mock for testing without a real TTY.
+
+```rust
+use ratatui::{backend::TestBackend, Terminal};
+
+#[test]
+fn test_widget_renders() {
+    let mut terminal = Terminal::new(TestBackend::new(80, 24)).unwrap();
+    
+    terminal.draw(|frame| {
+        frame.render_widget(MyWidget::new(), frame.area());
+    }).unwrap();
+    
+    terminal.backend().assert_buffer_lines([
+        "Expected line 1",
+        "Expected line 2",
+    ]);
+}
+```
+
+**Best for**: Unit testing widgets, layout logic, pure rendering functions.
+
+**Key Methods**:
+- `TestBackend::new(width, height)` - Create in-memory terminal
+- `backend.buffer()` - Access rendered buffer
+- `backend.assert_buffer(&expected)` - Assert buffer equality
+- `backend.assert_buffer_lines(lines)` - Assert against string lines
+
+---
+
+## 2. Snapshot Testing with `insta` (Visual Regression)
+
+Captures rendered output to `.snap` files; detects unexpected visual changes.
+
+**Official Recipe**: https://ratatui.rs/recipes/testing/snapshots/
+
+### Setup
+
+```toml
+[dev-dependencies]
+insta = "1.40.0"
+```
+
+```bash
+cargo install cargo-insta
+```
+
+### Usage Pattern
+
+```rust
+#[test]
+fn test_library_view_snapshot() {
+    let mut terminal = Terminal::new(TestBackend::new(80, 40)).unwrap();
+    let mut state = LibraryState::new(vec![/* test data */]);
+    
+    terminal.draw(|f| render_library(f, f.area(), &mut state)).unwrap();
+    
+    insta::assert_snapshot!(terminal.backend());
+}
+```
+
+### Workflow
+
+1. **First run**: Creates `.snap` file in `snapshots/` directory
+2. **Subsequent runs**: Compares against snapshot
+3. **Review changes**: `cargo insta review` - interactive diff viewer
+4. **Update snapshots**: `cargo insta test --review`
+
+**Best for**: Catching visual regressions, complex layouts, multi-component views.
+
+**Note**: Color/style information is not currently captured in snapshots.
+
+---
+
+## 3. Direct Unit Tests (State Logic, No Rendering)
+
+Test state machines and business logic without rendering.
+
+```rust
+#[test]
+fn test_library_navigation() {
+    let mut state = LibraryState::new(test_artists());
+    
+    state.move_down();
+    assert_eq!(state.selected_artist_index(), Some(1));
+    
+    state.focus_right();
+    assert_eq!(state.focus, LibraryFocus::Albums);
+}
+```
+
+**Best for**: State transitions, event handling logic, data transformations.
+
+---
+
+## 4. PTY-Based Integration Tests (`ratatui-testlib`)
+
+Real terminal emulation with event simulation for end-to-end testing.
+
+```toml
+[dev-dependencies]
+ratatui-testlib = "0.1"
+```
+
+```rust
+#[test]
+fn test_tab_switching_integration() -> Result<()> {
+    let mut harness = TuiTestHarness::new(80, 24)?;
+    harness.spawn(CommandBuilder::new("./ui-agregator"))?;
+    
+    harness.wait_for(|s| s.contents().contains("Library"))?;
+    harness.send_key(KeyCode::Char('2'))?;
+    harness.wait_for(|s| s.contents().contains("Wanted"))?;
+    
+    Ok(())
+}
+```
+
+**Best for**: Full E2E scenarios, keyboard/mouse flow testing, CI validation.
+
+---
+
+## 5. Property-Based Testing (`proptest`)
+
+Test invariants across random inputs - excellent for coordinate math and bounds checking.
+
+```toml
+[dev-dependencies]
+proptest = "1.4"
+```
+
+```rust
+proptest! {
+    #[test]
+    fn click_never_panics(x in 0u16..200, y in 0u16..100) {
+        let mut app = App::new();
+        app.size = Rect::new(0, 0, 80, 24);
+        app.handle_click(x, y, MouseButton::Left);
+    }
+}
+```
+
+**Best for**: Click handling, scroll bounds, layout calculations.
+
+---
+
+## Testing Plan for ui-agregator
+
+| Component | Strategy | Priority |
+|-----------|----------|----------|
+| `LibraryState` navigation | Unit tests | High |
+| `NotificationManager` TTL/history | Unit tests | High |
+| `App.handle_click()` routing | Unit + proptest | High |
+| Tab views (wanted, queue, history) | Snapshot tests (insta) | Medium |
+| `progress_bar.rs` | Already has 3 tests | Done |
+| Modals (help, quit) | Snapshot tests | Medium |
+| gRPC response → model conversion | Unit tests | Medium |
+| Full app E2E flows | ratatui-testlib | Low |
+
+---
+
+## Recommended Dependencies
+
+```toml
+[dev-dependencies]
+insta = "1.40.0"
+rstest = "0.22"           # Parameterized tests
+proptest = "1.4"          # Property-based testing
+# ratatui-testlib = "0.1" # Optional: PTY integration tests
+```
+
+---
+
+## Resources
+
+- **Official Testing Docs**: https://ratatui.rs/recipes/testing/
+- **Snapshot Testing Recipe**: https://ratatui.rs/recipes/testing/snapshots/
+- **TestBackend API**: https://docs.rs/ratatui/latest/ratatui/backend/struct.TestBackend.html
+- **insta Documentation**: https://insta.rs/docs/
+- **ratatui-testlib**: https://docs.rs/ratatui-testlib/