feat(tui): add color mapping module

Add color.rs with parse_color (string to ratatui::Color), level_fg (LogLevel to color via ColorConfig), and AVAILABLE_COLORS constant for the settings panel.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

crates/tui/src/color.rs

@@ -0,0 +1,76 @@
+use ratatui::style::Color;
+
+use log_viewer_core::config::ColorConfig;
+use log_viewer_core::types::LogLevel;
+
+#[allow(dead_code)]
+pub const AVAILABLE_COLORS: &[&str] = &[
+    "red", "green", "yellow", "blue", "magenta", "cyan", "gray", "white",
+];
+
+#[allow(dead_code)]
+pub fn parse_color(name: &str) -> Option<Color> {
+    name.parse::<Color>().ok()
+}
+
+#[allow(dead_code)]
+pub fn level_fg(level: Option<&LogLevel>, config: &ColorConfig) -> Option<Color> {
+    let color_name = match level? {
+        LogLevel::Error => &config.error,
+        LogLevel::Warn => &config.warn,
+        LogLevel::Info => &config.info,
+        LogLevel::Debug => &config.debug,
+        LogLevel::Trace => &config.trace,
+        LogLevel::Unknown(_) => &config.unknown,
+    };
+    parse_color(color_name)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_parse_color_valid() {
+        assert_eq!(parse_color("red"), Some(Color::Red));
+    }
+
+    #[test]
+    fn test_parse_color_invalid() {
+        assert_eq!(parse_color("not_a_color"), None);
+    }
+
+    #[test]
+    fn test_parse_color_darkgray() {
+        assert_eq!(parse_color("darkgray"), Some(Color::DarkGray));
+    }
+
+    #[test]
+    fn test_level_fg_none() {
+        let config = ColorConfig::default();
+        assert_eq!(level_fg(None, &config), None);
+    }
+
+    #[test]
+    fn test_level_fg_error() {
+        let config = ColorConfig::default();
+        assert_eq!(level_fg(Some(&LogLevel::Error), &config), Some(Color::Red));
+    }
+
+    #[test]
+    fn test_level_fg_all_levels() {
+        let config = ColorConfig::default();
+        assert_eq!(level_fg(Some(&LogLevel::Error), &config), Some(Color::Red));
+        assert_eq!(
+            level_fg(Some(&LogLevel::Warn), &config),
+            Some(Color::Yellow)
+        );
+        assert_eq!(level_fg(Some(&LogLevel::Info), &config), Some(Color::Green));
+        assert_eq!(level_fg(Some(&LogLevel::Debug), &config), Some(Color::Blue));
+        assert_eq!(level_fg(Some(&LogLevel::Trace), &config), Some(Color::Cyan));
+        assert_eq!(
+            level_fg(Some(&LogLevel::Unknown("custom".into())), &config),
+            Some(Color::Gray)
+        );
+    }
+}