feat(tui): add Span-based rendering with level colors and settings panel

Replace Line::styled with Span-based rendering: gutter (DarkGray fg) + content (level-based fg color). Add centered settings popup (S key) with j/k navigation, arrow key color cycling, number key shortcuts, Enter to save, Esc to cancel.

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

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

Cargo.lock

@@ -2305,6 +2305,7 @@ dependencies = [
  "regex",
  "serde",
  "serde_json",
+ "tempfile",
  "thiserror 2.0.18",
  "toml",
 ]
@@ -2328,6 +2329,7 @@ dependencies = [
  "log-viewer-core",
  "ratatui",
  "serde_json",
+ "tempfile",
 ]
 
 [[package]]