Slim down over-engineered code paths

- Replace parallel-du machinery with a single du process
- Drop hand-rolled syntax highlighter and raw-image stub plumbing
- Inline new_folder use case, merge TransferEvent::Done into Outcome
- Remove AppState.show_hidden mirror and unused favorites param
- Trim unused image crate features, drop stale plan docs

Author: ioma8

Cargo.toml

@@ -23,7 +23,7 @@ exclude = [
 base64 = "0.22.1"
 crossterm = "0.28.1"
 tui-input = { version = "0.10", features = ["crossterm"] }
-image = { version = "0.25.5", default-features = false, features = ["bmp", "gif", "ico", "jpeg", "png", "tiff", "webp"] }
+image = { version = "0.25.5", default-features = false, features = ["gif", "jpeg", "png"] }
 iterm2img = "0.1.0"
 libc = "0.2"
 little_exif = "0.6.2"