fix: replace stale cutoff with start/end in hourly filter

[SamzxSong]

Summary

applyFilter() in dashboard.py referenced an undefined cutoff variable when building hourlySrc, throwing ReferenceError and preventing every chart, total, and table on the dashboard from rendering.

cutoff was leftover from before #43 introduced start/end date-range bounds; #72 (hourly activity chart) was written against the older single-cutoff pattern and merged without the rename.

Change

• Mirror the daily filter pattern so hourly rows are gated by both start and end:

  // before
  (!cutoff || r.day >= cutoff)
  // after
  (!start || r.day >= start) && (!end || r.day <= end)

• Add a regression test (test_hourly_filter_uses_start_end_not_cutoff) that fails if the stale name returns.

Test plan

• New test fails before the fix, passes after
• python3 -m unittest tests.test_dashboard — all 24 tests pass
• Restart python3 cli.py dashboard and reload the page — Daily Token Usage, Average Hourly Distribution, By Model, Top Projects by Tokens, and Cost by Model all render