fix(cli): avoid panic on multi-byte UTF-8 in --since duration

[andrewwhitecdw]

Summary

openshell logs <name> --since <duration> panics when the duration's last character is multi-byte UTF-8 (e.g. --since 5€). parse_duration_to_ms split the input with split_at(s.len() - 1), indexing by byte length, which is not a char boundary for multi-byte characters. The CLI now returns the intended "unknown duration unit" error instead of crashing.

Related Issue

N/A — small fix found during code review (panic verified with a standalone repro: "5€".split_at(3) → byte index 3 is not a char boundary).

Changes

• parse_duration_to_ms splits off the last character using its UTF-8 length (char::len_utf8) instead of assuming a single byte
• Added unit tests for valid units (30s, 5m, 1h) and a regression test for multi-byte unit input

Testing

• mise run pre-commit passes (mise unavailable in this environment; ran equivalent cargo fmt + cargo clippy -p openshell-cli --all-targets — clean)
• Unit tests added/updated (cargo test -p openshell-cli parse_duration_to_ms — 2 passed)
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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