fix: replace undefined cutoff with start/end range in hourly filter

[q0821]

Summary

• Dashboard threw ReferenceError: cutoff is not defined at dashboard.py:622 (line 745 in source), breaking applyFilter() and stopping the page from rendering whenever loadData() fired.
• The reference was a leftover from the range-bounds refactor: the daily and session filters were migrated to use { start, end } from getRangeBounds(), but the hourly filter still referenced the old cutoff variable.
• Replaces the broken filter with the same start / end bounds the sibling filters use. As a side effect this also fixes a silent data-mismatch: under the broken code the (!cutoff || ...) short-circuit was always truthy, so the hourly chart was rendering the full timeline instead of the selected range — meaning the "Average Hourly Distribution" chart never honored upper bounds like prev-month, month, or week.

Repro (before fix)

1. Open the dashboard.
2. Browser console shows:

   ReferenceError: cutoff is not defined
       at (index):622:38
       at Array.filter (<anonymous>)
       at applyFilter ((index):621:53)
       at loadData ((index):1095:5)
   

3. Charts and tables do not populate.

Test plan

• Reload dashboard — no console error, charts and tables render.
• Switch between ranges (week, month, prev-month, 7d, 30d, 90d, all) — hourly chart updates in sync with the daily / model / project charts.
• Confirm grep -n cutoff dashboard.py returns no matches.