[codex] fix 7d report range to last7

[ericweichun]

Summary

• Map Project Usage 7d to a trailing last7 report window instead of calendar week.
• Keep week as the calendar-week report period for callers that want it explicitly.
• Add --last7 to the report CLI and update the analyzer tests to cover the split.

Test plan

• .venv/bin/python -m pytest tests/test_analyzer_pipeline.py tests/test_usage_cli.py tests/test_html_report_snapshot.py -q
• .venv/bin/ruff check menubar.py analyzer/reporter.py usage_cli.py tests/test_analyzer_pipeline.py tests/test_usage_cli.py
• .venv/bin/mypy menubar.py analyzer/reporter.py usage_cli.py tests/test_analyzer_pipeline.py tests/test_usage_cli.py
• Manually verified in menu bar mode / TUI mode (if UI-related)

Notes for reviewer

• This keeps the Project Usage label semantics aligned with the UI: 7d is recent seven days, while week remains a calendar-week period for any existing explicit callers.
• The new last7 period gets the same comparison treatment as week and month, so the report still shows a previous-window comparison for seven-day ranges.