feat(tui): add AppMode, level_cache and config integration

Add AppMode enum (Normal/Settings), level_cache field populated in recompute_wrap_cache via detect_level(), color_config loaded from TOML in main.rs. Refactor handle_key for mode dispatch with S key to enter settings.

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

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

crates/tui/src/main.rs

@@ -1,6 +1,7 @@
 use clap::Parser;
 
 mod app;
+mod color;
 mod ui;
 
 #[derive(Parser)]
@@ -20,6 +21,7 @@ fn main() -> anyhow::Result<()> {
     let mut terminal = ratatui::Terminal::new(backend)?;
 
     let mut app = app::App::new();
+    app.color_config = log_viewer_core::config::ColorConfig::load();
 
     if let Some(file) = cli.files.first()
         && let Err(e) = app.load_file(file)