Add visual TUI test coverage

tests/helpers/snapshot_utils.rs

@@ -2,7 +2,7 @@
 
 use ratatui::backend::TestBackend;
 use ratatui::buffer::Buffer;
-use ratatui::layout::Rect;
+use ratatui::style::{Color, Modifier};
 use ratatui::Terminal;
 
 /// Конвертирует Buffer в читаемую строку для snapshot тестов
@@ -25,6 +25,64 @@ pub fn buffer_to_string(buffer: &Buffer) -> String {
     result
 }
 
+/// Serializes only cells with non-default style, grouped by row and style.
+pub fn buffer_to_style_snapshot(buffer: &Buffer) -> String {
+    let area = buffer.area();
+    let mut rows = Vec::new();
+
+    for y in 0..area.height {
+        let mut segments = Vec::new();
+        let mut x = 0;
+
+        while x < area.width {
+            let cell = &buffer[(x, y)];
+            if is_default_style(cell) {
+                x += 1;
+                continue;
+            }
+
+            let start = x;
+            let fg = cell.fg;
+            let bg = cell.bg;
+            let modifier = cell.modifier;
+            let mut text = String::new();
+
+            while x < area.width {
+                let next = &buffer[(x, y)];
+                if is_default_style(next)
+                    || next.fg != fg
+                    || next.bg != bg
+                    || next.modifier != modifier
+                {
+                    break;
+                }
+                text.push_str(next.symbol());
+                x += 1;
+            }
+
+            segments.push(format!(
+                "{}..{} {:?}/{:?}/{:?}: {:?}",
+                start,
+                x.saturating_sub(1),
+                fg,
+                bg,
+                modifier,
+                text.trim_end()
+            ));
+        }
+
+        if !segments.is_empty() {
+            rows.push(format!("y={}: {}", y, segments.join(" | ")));
+        }
+    }
+
+    rows.join("\n")
+}
+
+fn is_default_style(cell: &ratatui::buffer::Cell) -> bool {
+    cell.fg == Color::Reset && cell.bg == Color::Reset && cell.modifier == Modifier::empty()
+}
+
 /// Создаёт TestBackend с заданным размером и рендерит UI
 pub fn render_to_buffer<F>(width: u16, height: u16, render_fn: F) -> Buffer
 where
@@ -52,6 +110,7 @@ macro_rules! assert_ui_snapshot {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use ratatui::layout::Rect;
     use ratatui::widgets::{Block, Borders};
 
     #[test]