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>

Cargo.toml

@@ -23,3 +23,4 @@ crossterm = "0.29"
 clap = { version = "4", features = ["derive"] }
 log-viewer-core = { path = "crates/core" }
 textwrap = "0.16"
+tempfile = "3"