feat: add independent interval control for session histogram

[sanggggg]

Summary

Adds independent interval control to the session activity histogram, allowing users to customize the granularity of time-based data visualization separately from the duration selection.

Changes

• New interval selector UI: Added dropdown with 5 interval options (30m, 1h, 6h, 1d, 1w) next to the existing duration selector
• Smart interval filtering: Automatically hides interval options that are longer than the selected duration (e.g., for "Last 6 hours", only 30m and 1h are available)
• Auto-reset logic: When duration changes and the current interval becomes invalid, automatically resets to the largest valid interval
• Centered histogram bars: Offset timestamps by half the interval duration to align bar centers with x-axis tick marks instead of left edges
• Updated defaults: Changed default view from 24h/15min to 30d/1d for better overview of long-term activity
• Refactored configuration: Separated TIME_RANGE_CONFIG from interval settings for independent control

Interval Availability Matrix

Duration	Available Intervals
Last 6 hours	30m, 1h
Last 24 hours	30m, 1h, 6h
Last 7 days	30m, 1h, 6h, 1d
Last 30 days	30m, 1h, 6h, 1d, 1w

Technical Details

• Implemented availableIntervals computed value that filters options based on duration
• Added useEffect hook to handle interval validation when duration changes
• Modified timestamp formatting logic to adapt labels based on interval granularity
• Timestamp centering: centeredTime = originalTime + (intervalMinutes / 2)

Testing

• ✅ All Biome formatting and linting checks pass
• ✅ Interval selector shows correct options for each duration
• ✅ Auto-reset works when switching from longer to shorter durations
• ✅ Charts render correctly with all interval/duration combinations

🤖 Generated with Claude Code