This PR implements the income and spending insight cards for the dashboard.

Requirements

• Should generate a time series of income and spending for a Family, scoped to Period.last_30_days
  • Each data point in the series represents a “Trailing 30 day sum” of income (negative transaction values)
  • The “headline” number shows the last value in the series
  • The “trend” value is the overall time series trend
• Should find the top 3 accounts with the most income or spending within the last 30 days and display the sum
  • Each account is an anchor that goes to that account page
  • Bank icons are not in-scope (just use a generic letter as shown in designs)
  • A “more” counter should show if there are more than 3 accounts
• Display a trendline of series. No tooltips, no axes.
• These insights should be included in the dashboard

/claim #645 Resolves #645