Add --html flag for email-friendly HTML table output

[Copilot]

Plain-text column alignment in the report output breaks in proportional-font email clients. Adding --html renders the same data as inline-CSS HTML tables that look correct in any email client.

New functions

• _collect_meta_keys(entries) — ordered union of metadata field names across all entries (required fields first)
• _entry_to_html_row(...) — single directory entry → <tr>, all values passed through html.escape()
• _print_html_entries(entries) — full <table> with adaptive columns (Groups, Files, Total Size omitted when absent) plus any metadata columns
• _print_html_report(report_data) — --report summary as two tables: per-track counts and per-user on-track shares with a total-average footer row

Wiring

• main() gains html_output: bool = False; selects HTML renderers over plain-text ones when set. --output (YAML to file) takes precedence when both flags are given.
• cli() gains --html argument.

Usage

# Directory listing
python3 ontrack.py --config ontrack.config --html

# On-track summary
python3 ontrack.py --config ontrack.config --report --html

# Pipe to mailer
python3 ontrack.py --config ontrack.config --html \
  | mail -a "Content-Type: text/html" -s "ontrack report" team@lab.org

No new dependencies — uses stdlib html.escape() throughout.