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/Cargo.toml

@@ -10,3 +10,6 @@ clap.workspace = true
 anyhow.workspace = true
 log-viewer-core.workspace = true
 serde_json.workspace = true
+
+[dev-dependencies]
+tempfile = { workspace = true }