fix: use start/end bounds in hourly filter instead of undefined cutoff

[tswicegood]

Summary

The dashboard throws a JS error on every load:

(index):1097 ReferenceError: cutoff is not defined
    at (index):622:38
    at Array.filter (<anonymous>)
    at applyFilter ((index):621:53)
    at loadData ((index):1095:5)

The hourly aggregation filter in applyFilter() references a cutoff variable that doesn't exist in scope — it looks like a leftover from an earlier iteration. The surrounding daily/sessions filters use start / end from getRangeBounds(selectedRange). This PR switches the hourly filter to the same bounds so it matches the rest of the code and the error goes away.

• One-line change in dashboard.py
• Hourly data is now range-filtered consistently with the daily and sessions filters (both lower and upper bound)

Test plan

• Open the dashboard — no console error on load
• Switch ranges (week, month, prev-month, 7d, 30d, 90d, all) — Average Hourly Distribution chart updates and only includes hours from the selected range
• Toggle Local / UTC — hourly chart re-buckets correctly

🤖 Generated with Claude Code

[tswicegood]

Just realized this is a duplicate of #117.