feat(stats): demote reading source in default charts and breakdowns

Reading source is often less relevant for day-to-day analytics than
book format or reading pace. Shift the source distribution chart to the
end of the user dashboard and change the default breakdown dimension
from source to format to prioritize more useful metrics. Include a
migration to update existing users who have not customized their chart
order.

packages/types/src/statistics.ts

@@ -79,7 +79,6 @@ export const DEFAULT_LIBRARY_CHART_ORDER: StatisticsChartId[] = [
 
 export const DEFAULT_USER_CHART_ORDER: StatisticsChartId[] = [
   "reading-heatmap",
-  "reading-source-distribution",
   "peak-reading-hours",
   "favorite-reading-days",
   "completion-timeline",
@@ -92,6 +91,7 @@ export const DEFAULT_USER_CHART_ORDER: StatisticsChartId[] = [
   "reading-clock",
   "reading-session-timeline",
   "session-archetypes",
+  "reading-source-distribution",
 ];
 
 export const DEFAULT_STATISTICS_CHART_ORDER: StatisticsChartId[] = [...DEFAULT_LIBRARY_CHART_ORDER, ...DEFAULT_USER_CHART_ORDER];