fix(tui): handle settings save errors and rewrite truncate_to_columns (closes #30)

- Report save failures in settings instead of silently discarding errors
- Save draft first, commit color_config only on success (prevents split-brain)
- Show error message in red on status bar, stay in Settings mode on failure
- Clear error on Esc, entering settings, or any settings edit
- Rewrite truncate_to_columns as dedicated O(prefix) truncator
- Tab: stop before tab if expansion would exceed width
- Fixes pre-existing test_truncate_to_columns_tab failure

Cargo.lock

@@ -2358,6 +2358,7 @@ dependencies = [
  "ratatui",
  "serde_json",
  "tempfile",
+ "unicode-width",
 ]
 
 [[package]]