Add time tracking statistics for flashcard reviews

[Copilot]

The plugin lacked time-based analytics. RemNote SDK exposes responseTime in card.repetitionHistory but it wasn't being utilized.

Changes

Data Processing (src/lib/dataProcessing.ts)

• getTimeSpentPerDay(): Aggregates responseTime by day with review counts
• calculateTimeStatsSummary(): Computes total time, averages (per day/study day/card), cards/minute rate, study day percentage
• formatTime(): Multi-format time formatter (short: "1h 23m", hours: "1.38h", seconds: "34.39s")

UI Components (src/widgets/statistics.tsx)

• Time Spent metric card: 5th card in Review Statistics grid showing total time + daily average (amber color)
• Time spent chart: Mixed bar + area chart with dual y-axes
  • Daily time bars (left axis): Amber bars showing time spent per day in minutes
  • Cumulative time area (right axis): Blue gradient area overlay showing total accumulated time in hours
  • Summary panel: 4 metrics (total time, avg per day, avg per study day, avg per card) with consistent stat-card styling
  • Legend: Shows both "Daily Time" and "Cumulative Time" series
  • Interactive tooltips: Different formatters for daily ("X.X minutes (N reviews)") and cumulative ("X.XX hours total") data

// Example usage
const timeData = getTimeSpentPerDay(cards, startDate, endDate);
const summary = calculateTimeStatsSummary(timeData, startDate, endDate);
// summary: { totalTimeMs, averageTimePerCard, cardsPerMinute, ... }

Implementation Notes

• Grid layout updated from lg:grid-cols-4 to lg:grid-cols-5 for new metric card
• Summary panel styling matches Review Statistics section (stat-card class, consistent text sizes, spacing)
• Chart uses dual y-axes: left for daily minutes, right for cumulative hours
• Area chart with gradient fill (opacity 0.5 to 0.1) and smooth curve
• Gracefully handles missing responseTime (optional field in SDK)
• Uses Map for O(n) daily aggregation
• Follows existing chart styling patterns

Documentation

Updated README with v1.6.0 changelog entry and feature descriptions.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Feature Request: Statistics related to time spent reviewing</issue_title>
<issue_description>Thanks for the terrific improvements you've brought us recently!

One thing that I miss are stats relating to total time spent reviewing flashcards. As we have in the repetitionHistory (card namespace) the responseTime, this would not be difficult. I don't know exactly the best way to show it, but an entry with the total review time in the Review Statistics section and also something like we have in Anki would be great:

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Feature Request: Statistics related to time spent reviewing #16

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[Justtolook]

see comments for changes

[Justtolook]

keep consistent styling (see review statistics as comparison)

[Copilot]

Updated in f65c53e. The summary panel now uses consistent styling with Review Statistics section:

• Changed to background-primary (matches stat cards)
• Added stat-card class
• Text sizes: text-[10px] md:text-xs uppercase tracking-wide
• Font: text-lg md:text-xl font-bold
• Spacing: mb-1 md:mb-2, gap-3 md:gap-4

[Justtolook]

add a second data series to this chart that shows the cumulative time spent as an area that uses the right axis

[Copilot]

Added in f65c53e. The chart now displays:

• Daily time bars (amber, left axis in minutes)
• Cumulative time area (blue gradient, right axis in hours)
• Smooth curve overlay showing total accumulated time
• Gradient fill with opacity 0.5 to 0.1
• Legend at top showing both series
• Dual tooltips: "X.X minutes (N reviews)" for bars, "X.XX hours total" for area