Function detect_icon_mode 

pub fn detect_icon_mode() -> IconMode

Detects the current icon mode.

Uses the configured detector (default: auto-detect via NERD_FONT env var). Always returns a resolved mode (Classic or NerdFont), never Auto.

The detector can be overridden via set_icon_detector for testing.

Returns

• IconMode::NerdFont if Nerd Font is detected/configured
• IconMode::Classic otherwise