Box-drawing chars (U+2500), em dashes (U+2014), and en dashes (U+2013)
in comments, section dividers, log messages, and docstrings are not
representable on legacy code pages like CP1252. Replace them with plain
ASCII dashes so the codebase is consistently ASCII-safe.
User-facing UI strings (placeholders, separators, display text in the
frontend) are left unchanged since they render in the browser which
handles Unicode natively.
When multiple image columns are found, probes them (HEAD for URLs,
os.path.exists for paths) and picks the first that works.
Skips probing when top candidate is PIL/dict (score >= 75).
find_image_column now scores candidates by resolvability (PIL > dict > URL > path)
and has a Pass 2 value-based fallback for columns not matching image keywords.
Fixes phiyodr/coco2017 picking file_name (unresolvable) over coco_url (resolvable).
- Detect and convert ShareGPT/ChatML conversations with <image> placeholders
- Add file_name/filename as image column keywords
- Detect image paths and URLs by value (string ending in .jpg/.png/etc)
